@booklet {846, title = {Experimental results for Evaluating User Similarity Metrics in Sparse Collaborative Filtering Datasets (version 2)}, year = {2025}, abstract = {

In this technical report, we present the experimental findings from evaluating 15 user similarity metrics when applied in sparse collaborative filtering (CF) datasets.

}, keywords = {collaborative filtering, Evaluation, Sparse Datasets, User similarity metrics}, url = {https://soda.dit.uop.gr/sites/soda.dit.uop.gr/files/publications/technical-reports/soda-TR-24001-v2.pdf}, author = {Kiriakos Sgardelis and Dionisis Margaris and Dimitris Spiliotopoulos and Costas Vassilakis} } @conference {838, title = {Estimating MPdist with SAX and Machine Learning}, booktitle = {New Trends in Database and Information Systems (Proceedings of ADBIS 2024, Short Papers, Workshops, Doctoral Consortium and Tutorials)}, year = {2024}, publisher = {Springer Cham}, organization = {Springer Cham}, abstract = {

MPdist is a distance measure which considers two time series to be similar if they share many similar subsequences. However, computing MPdist can be slow, especially for large time series. We propose a technique for the approximate computation of MPdist that uses the SAX representation of the time series to quickly estimate the Nearest Neighbor (NN) distance of each subsequence, and then applies a Machine Learning model to correct the accuracy loss incurred. Our method is orders of magnitude faster than the exact computation of MPdist; at the same time, our best approximation computes the NN of a time series with high accuracy. A thorough evaluation of our technique is provided.

}, keywords = {Distance metric, Machine Learning, MPdist, Random Forest, SAX, Time series}, doi = {https://doi.org/10.1007/978-3-031-70421-5_32}, author = {Mihalis Tsoukalos and Pantelis Chronis and Nikos Platis and Costas Vassilakis}, editor = {Joe Tekli and Johann Gamper and Richard Chbeir and Yannis Manolopoulos and Salma Sassi and Mirjana Ivanovic and Vargas-Solar, Genoveva and Ester Zumpano} } @article {833, title = {Experimental results for Evaluating User Similarity Metrics in Sparse Collaborative Filtering Datasets}, number = {TR-24001}, year = {2024}, month = {03/2024}, institution = {SODA lab}, type = {Technical Report}, address = {Tripolis, Greece}, keywords = {collaborative filtering, Evaluation, Sparse Datasets, User similarity metrics}, issn = {SODA-TR-24001}, author = {Kiriakos Sgardelis and Dionisis Margaris and Dimitris Spiliotopoulos and Costas Vassilakis} } @article {841, title = {Exploiting Rating Prediction Certainty for Recommendation Formulation in Collaborative Filtering}, journal = {Big Data and Cognitive Computing}, volume = {8}, year = {2024}, pages = {53}, abstract = {

Collaborative filtering is a popular recommender system (RecSys) method that produces rating prediction values for products by combining the ratings that close users have already given to the same products. Afterwards, the products that achieve the highest prediction values are recommended to the user. However, as expected, prediction estimation may contain errors, which, in the case of RecSys, will lead to either not recommending a product that the user would actually like (i.e., purchase, watch, or listen) or to recommending a product that the user would not like, with both cases leading to degraded recommendation quality. Especially in the latter case, the RecSys would be deemed unreliable. In this work, we design and develop a recommendation algorithm that considers both the rating prediction values and the prediction confidence, derived from features associated with rating prediction accuracy in collaborative filtering. The presented algorithm is based on the rationale that it is preferable to recommend an item with a slightly lower prediction value, if that prediction seems to be certain and safe, over another that has a higher value but of lower certainty. The proposed algorithm prevents low-confidence rating predictions from being included in recommendations, ensuring the recommendation quality and reliability of the RecSys.

}, keywords = {Algorithm, collaborative filtering, confidence, Evaluation, Rating Predictions, Recommender Systems, Uncertainty}, issn = {2504-2289}, doi = {10.3390/bdcc8060053}, author = {Dionisis Margaris and Sgardelis, Kiriakos and Spiliotopoulos, Dimitris and Costas Vassilakis} } @conference {826, title = {EnQuest: A Cloud Datalake Infrastructure for Heterogeneous Analytics in Maritime and Tourism Domains}, booktitle = {DATA 2023 (demo)}, year = {2023}, abstract = {

Within recent years, transportation and supply chain have been affected by an avalanche of data including IoT devices, online services, social media and others. However, the implemented solutions so far are fragmented, physically distributed, require specialized IT knowledge to deploy, and involve significant IT experience to operate even for trivial tasks. In the context of the ENIRISST+ project, we developed EnQuest1, an online data lake that allows users without any IT background to harvest, store, organize, analyze and share heterogeneous, multi-faceted maritime/tourism data. EnQuest provides a zero-administration, zero-cost, integrated framework that enables scientists, practitioners and other stakeholders within the above domains to easily: (i) deploy data acquisition services (crawlers, dataset imports, questionnaire forms), (ii) design and manage versatile customizable data stores, (iii) share whole datasets or horizontal/vertical data shards with other stakeholders, (iv) search, filter and analyze data via an expressive yet simple-to-use graphical query engine and visualization tools, and (v) perform user management and access control operations on the stored data. To the best of our knowledge, this is the first approach in the literature that targets users without an IT background and focuses on collecting, managing, analyzing, and sharing diverse, multi-faceted data in maritime the and tourism sectors. The main goals of the proposed demonstration are to showcase the EnQuest system to the conference participants and promote the work of the ENIRISST+ project consortium. During the demo we will (i) deploy social media crawlers to gather maritime data and show the participants how to process and visualise them, (ii) create live questionnaires using the EnQuest functionality, ask the participants to fill them in and demonstrate the live processed data on appropriate dashboards, and (iii) demonstrate how to import data and subscribe to data flows of other users through the implemented publish/subscribe module. For the needs of the demo we will only require internet connection and a desk; we will have our own laptops to use at the demonstration.

}, keywords = {big data, cloud systems, data analytics, datalakes, maritime, tourism}, author = {Kimon Deligiannis and Christos Tryfonopoulos and Paraskevi Raftopoulou and Nikos Platis and Costas Vassilakis} } @conference {829, title = {Estimating \iSAX Parameters for Efficiency}, booktitle = {New Trends in Database and Information Systems (Proceedings of the 2023 European Conference on Advances in Databases and Information Systems / ADBIS 2023)}, year = {2023}, publisher = {Springer Nature Switzerland}, organization = {Springer Nature Switzerland}, address = {Cham}, abstract = {

\iSAX is considered one of the most efficient indexes for time series. Several parameters affect the construction of an \iSAX index: the sliding window size, the threshold value, the number of segments and the maximum cardinality, the last two being related to the SAX representation. In this paper (i) we consider the effect of each parameter on the efficiency of the \iSAX index, (ii) we evaluate the importance of each parameter, and, (iii) suggest how to optimize these parameters.

}, keywords = {Indexing, Parameter tuning, SAX, Time series}, isbn = {978-3-031-42941-5}, doi = {10.1007/978-3-031-42941-5_1}, author = {Tsoukalos, Michalis and Platis, Nikos and Costas Vassilakis} } @conference {825, title = {Estimating \iSAX Parameters for Efficiency}, booktitle = {Proceedings of the 27th European Conference on Advances in Databases and Information Systems (ADBIS 2023)}, year = {2023}, abstract = {

\iSAX is considered one of the most efficient indexes for time series. Several parameters affect the construction of an \iSAX index: the sliding window size, the threshold value, the number of segments and the maximum cardinality, the last two being related to the SAX representation. In this paper (i) we consider the effect of each parameter on the efficiency of the \iSAX index, (ii) we evaluate the importance of each parameter, and, (iii) suggest how to optimize these parameters.

}, keywords = {Indexing, Parameter tuning, SAX, Time series}, author = {Tsoukalos, Michalis and Platis, Nikos and Costas Vassilakis} } @article {811, title = {The Effects of Logistics Websites{\textquoteright} Technical Factors on the Optimization of Digital Marketing Strategies and Corporate Brand Name}, journal = {Processes}, volume = {10}, year = {2022}, month = {may}, pages = {892}, keywords = {advertising, big data, brand name, competitive advantage, digital marketing, logistics, predictive model, SEM, user engagement, web analytics}, doi = {https://doi.org/10.3390/pr10050892}, url = {https://www.mdpi.com/2227-9717/10/5/892}, author = {Damianos P. Sakas and Dimitrios P. Reklitis and Panagiotis Trivellas and Costas Vassilakis and Marina C. Terzi} } @mastersthesis {855, title = {Emotion analysis for videos with Greek speech}, year = {2021}, month = {10/2021}, pages = {44}, type = {Undergraduate}, abstract = {

Opinions play a major role to almost all human activities, because they are key influencers of our choices and way of thinking. Whenever we need to make a decision, we want to know others{\textquoteright} opinions on the subject at hand. In the real world, businesses
and organizations want to know consumer or public opinions about their products and services. On the other hand, individual consumers also want to know the opinions of
other people that have used a particular product, before purchasing it.


With the explosive growth of social networks (e.g. blogs, micro-blogs, forum, reviews, videos, comments and postings in social network sites) on the Web, a huge dataset has
been created, ready to be exploited to find user preferences and opinions. Sentiment analysis focuses on the task of automatically identifying whether a piece of text
expresses a positive or negative polarity about the subject matter. In this thesis, sentiment analysis is performed on Greek review and unboxing videos that are related
to tech products (mostly smartphones and laptops), using a sentiment lexicon. In addition, previous work in this area on Greek and the English languages is examined and compared.

}, author = {Sing Manvir and Costas Vassilakis} } @article {788, title = {Examining the use of STEAM Education in Preschool Education}, journal = {European Journal of Engineering Research and Science}, volume = {TBD}, year = {2020}, month = {TBD}, abstract = {

STEAM (Science, Technology, Engineering, Arts and Mathematics) initiatives are of current interest for both in-school and out-of-school contexts in North America. This is a new concept which is shifting educational paradigms towards art integration in STEM subjects. This article focuses on the need for STEAM education at the early childhood level and investigates the teaching and educational models of STEAM education in the kindergarten and in the first grades of primary school and how pre-school and primary school teachers see these models in the Greek context but also what students eventually learn from these models. The purpose of this chapter is to better understand the educational programs of STEAM education, which are offered by non-profit organizations and both public and private schools. Preschool children have a natural disposition toward science with their sense of curiosity and creativity. More research needs to be done in the area of STEAM implementation in the K-8 classrooms to incorporate engineering education.

}, keywords = {Early science education, Greek STEAM implementation, K-8 STEAM, STEAM educational models, STEM/STEAM education}, author = {C. Spyropoulou and M. Wallace and Costas Vassilakis and V. Poulopoulos} } @conference {lykourentzou_2019_crops, title = {Exploring Self-organisation in Crowd Teams}, booktitle = {Proceedings of the Crowd-Powered e-Services (CROPS) Workshop in the context of the 19th IFIP Conference on e-Business, e-Services and e-Society (I3E 2019)}, year = {2020}, month = {01/2020}, abstract = {

Online crowds have the potential to do more complex work in teams, rather than as individuals. Team formation algorithms typically maximize some notion of global utility of team output by allocating people to teams or tasks. However, decisions made by these algorithms do not consider the decisions or preferences of the people themselves. This paper explores a complementary strategy, which relies on the crowd itself to self-organize into eective teams. Our preliminary results show that users perceive the ability to choose their teammate extremely useful in a crowdsourcing setting. We also nd that self-organisation makes users feel more productive, creative and responsible for their work product.

}, keywords = {computer-supported collaboration, creative writing, crowd teams, self-organization}, doi = {10.1007/978-3-030-39634-3_15}, author = {Ioanna Lykourentzou and Antonios Liapis and Costas Papastathis and Konstantinos Papangelis and Costas Vassilakis} } @article {SODA-TR-19003, title = {Experimental results for considering Item Adoption Eagerness Information in Collaborative Filtering{\textquoteright}s Rating Prediction}, year = {2019}, month = {07/2019}, publisher = {Software and Database Systems Lab, University of the Peloponnese}, type = {Technical report}, address = {Tripolis}, abstract = {In this technical report, we present the experimental findings from applying an algorithm that considers Item Adoption Eagerness Information in the rating prediction formulation process, in order to increase rating prediction quality in Collaborative Filtering (CF). To this end, the algorithm is applied to seven datasets, which are widely used in recommender system (RS) research. The results indicate that the above algorithm introduces considerable prediction accuracy gains. }, url = {https://soda.dit.uop.gr/?q=TR-19003}, author = {Dionisis Margaris and Dimitris Spiliotopoulos and Costas Vassilakis} } @conference {poulopoulos2018a, title = {The Effect of Social Media Trending Topics Related to Cultural Venues{\textquoteright} Content}, booktitle = {2018 13th International Workshop on Semantic and Social Media Adaptation and Personalization (SMAP)}, year = {2018}, month = {sep}, publisher = {IEEE}, organization = {IEEE}, abstract = {Social media have gained the majority of attention on the Internet having an extreme number of daily visitors worldwide. The amount of information exchanged is vast, while users have become equally producers and consumers of data. Words like {\textquotedblleft}trending{\textquotedblright}, {\textquotedblleft}influencers{\textquotedblright}, {\textquotedblleft}likes{\textquotedblright} and {\textquotedblleft}viral{\textquotedblright} are in the daily agenda of data analyzers, as they are associated with factors that play an important role in the influence of social media content to its audience. These aspects are nowadays strongly taken into account by organizations that want to draw the public{\textquoteright}s attention to the content they deliver, and in this context cultural institutions have already started to take under great consideration not only their presence in social media, but also the exploitation of social media dynamics. In this paper we propose a method that can enable cultural venues to benefit from matches between their own content and ongoing discussions on social media. More specifically we extract trending topics that can be related semantically with the content of a cultural institute and examine how a venue can benefit by exploiting these matches. The proposed algorithm has been developed in the scope of the {\textquotedblleft}CrossCult{\textquotedblright} H2020 project, and has been experimentally tested by analyzing the case of Twitter in Greek language.}, keywords = {cultural informatics, cultural venues, personalization, social media, trending topics}, doi = {10.1109/SMAP.2018.8501878}, author = {Vassilis Poulopoulos and Costas Vassilakis and Manolis Wallace and Angela Antoniou and Giorgos Lepouras} } @article {margaris2018tldks, title = {Enhancing Rating Prediction Quality Through Improving the Accuracy of Detection of Shifts in Rating Practices}, journal = {Lecture Notes in Computer Science}, volume = {10940 (LNCS)}, number = {XXXVII}, year = {2018}, pages = {151{\textendash}191}, publisher = {Springer Berlin Heidelberg}, abstract = {The most widely used similarity metrics in collaborative filtering, namely the Pearson Correlation and the Adjusted Cosine Similarity, adjust each individual rating by the mean of the ratings entered by the specific user, when computing similarities, due to the fact that users follow different rating practices, in the sense that some are stricter when rating items, while others are more lenient. However, a user{\textquoteright}s rating practices change over time, i.e. a user could start as lenient and subsequently become stricter or vice versa; hence by relying on a single mean value per user, we fail to follow such shifts in users{\textquoteright} rating practices, leading to decreased rating prediction accuracy. In this work, we present a novel algorithm for calculating dynamic user averages, i.e. time-in-point averages that follow shifts in users{\textquoteright} rating practices, and exploit them in both user-user and item-item collaborative filtering implementations. The proposed algorithm has been found to introduce significant gains in rating prediction accuracy, and outperforms other dynamic average computation approaches that are presented in the literature.}, doi = {10.1007/978-3-662-57932-9_5}, author = {Dionisis Margaris and Costas Vassilakis} } @conference {mathas2018a, title = {Evaluation of Apache Spot\textquotesingles machine learning capabilities in an SDN/NFV enabled environment}, booktitle = {Proceedings of the 13th International Conference on Availability, Reliability and Security - ARES 2018}, year = {2018}, publisher = {ACM Press}, organization = {ACM Press}, abstract = {Software Defined Networking (SDN) and Network Function Virtualisation (NFV) are transforming modern networks towards a service-oriented architecture. At the same time, the cybersecurity industry is rapidly adopting Machine Learning (ML) algorithms to improve detection and mitigation of complex attacks. Traditional intrusion detection systems perform signature-based detection, based on well-known malicious traffic patterns that signify potential attacks. The main drawback of this method is that attack patterns need to be known in advance and signatures must be preconfigured. Hence, typical systems fail to detect a zero-day attack or an attack with unknown signature. This work considers the use of machine learning for advanced anomaly detection, and specifically deploys the Apache Spot ML framework on an SDN/NFV-enabled testbed running cybersecurity services as Virtual Network Functions (VNFs). VNFs are used to capture traffic for ingestion by the ML algorithm and apply mitigation measures in case of a detected anomaly. Apache Spot utilises Latent Dirichlet Allocation to identify anomalous traffic patterns in Netflow, DNS and proxy data. We evaluate the overall performance of Apache Spot by deploying Denial of Service (Slowloris, BoNeSi) and a Data Exfiltration attack (iodine).}, keywords = {Apache Spot, Latent Dirichlet Allocation, Machine Learning, Network Function Virtualisation, Penetration Testing, SHIELD Project�, Software Defined Networking}, doi = {10.1145/3230833.3233278}, author = {Christos M. Mathas and Olga E. Segou and Georgios Xylouris and Dimitris Christinakis and Michail-Alexandros Kourtis and Costas Vassilakis and Anastasios Kourtis} } @conference {PoulopoulosGIIS2018, title = {ExhiSTORY: IoT in the service of Cultural Heritage}, booktitle = {Proceedings of the Global Information Infrastructure and Networking Symposium (GIIS 2018), Special Session on IoT and Cultural Heritage Protection}, year = {2018}, month = {oct}, publisher = {IEEE}, organization = {IEEE}, abstract = {Creating stories for exhibitions is a fascinating and in parallel laborious task. As every exhibition is designed to tell a story, museum curators are responsible for analyzing each exhibit in order to extract messages that form a story and position accordingly the objects in correct order within the museum space. In this context, we analyze how the technological advances in the fields of sensors and the Internet of Things can be utilized in order to construct a {\textquotedblleft}smart space{\textquotedblright}, where exhibits can communicate with the visitors and to each other and can be organized automatically so that they can generate rich, personalized, coherent and highly stimulating experiences. We present the architecture of the system named {\textquotedblleft}exhiSTORY{\textquotedblright}, that intends to provide the appropriate infrastructure to be used in museums and places where exhibitions are held, in order to support smart exhibits. We discuss and analyze the architecture of the system and the ways of its application in a cultural space.}, doi = {10.1109/GIIS.2018.8635759}, author = {Vassilis Poulopoulos and Costas Vassilakis and Angela Antoniou and Manolis Wallace and Giorgos Lepouras and Martin Lopez Nores} } @article {Vassilakis_fgcs17, title = {exhiSTORY: Smart Exhibits That Tell Their Own Stories}, journal = {Future Generation of Computer Systems}, volume = {81}, year = {2018}, pages = {542-556}, abstract = {Museum exhibitions are designed to tell a story; this story is woven by curators and in its context a particular aspect of each exhibit, fitting to the message that the story is intended to convey, is highlighted. Adding new exhibits to the story requires curators to identify for each exhibit its aspects that fit to the message of the story and position the exhibit at the right place in the story thread. The availability of rich semantic information for exhibits, allows for exploiting the wealth of meanings that museum exhibits express, enabling the automated or semi-automated generation of practically countless stories that can be told. Personalization algorithms can then be employed to choose from these stories the ones most suitable for each individual user, based on the semantics of the stories and information within the user profile. In this work we examine how opportunities arising from technological advances in the fields of IoT and semantics can be used to develop smart, self-organizing exhibits that cooperate with each other and provide visitors with comprehensible, rich, diverse, personalized and highly stimulating experiences. These notions are included in the design of a system named exhiSTORY, which also exploits previously ignored information and identifies previously unseen semantic links. We present the architecture of the system and discuss its application potential.}, keywords = {Cultural heritage, IoT, personalization, Quality of Experience (QoE), Self-organizing exhibitions, semantics}, doi = {https://doi.org/10.1016/j.future.2017.10.038}, url = {https://www.sciencedirect.com/science/article/pii/S0167739X17324093}, author = {Costas Vassilakis and Vassilis Poulopoulos and Angela Antoniou and Manolis Wallace and Giorgos Lepouras and Martin Lopez Nores} } @article {margaris2018a, title = {Exploiting Rating Abstention Intervals for Addressing Concept Drift in Social Network Recommender Systems}, journal = {Informatics}, volume = {5}, number = {2}, year = {2018}, month = {apr}, pages = {21 pages}, abstract = {One of the major problems that social networks face is the continuous production of successful, user-targeted information in the form of recommendations, which are produced exploiting technology from the field of recommender systems. Recommender systems are based on information about users{\textquoteright} past behavior to formulate recommendations about their future actions. However, as time goes by, social network users may change preferences and likings: they may like different types of clothes, listen to different singers or even different genres of music and so on. This phenomenon has been termed as concept drift. In this paper: (1) we establish that when a social network user abstains from rating submission for a long time, it is a strong indication that concept drift has occurred and (2) we present a technique that exploits the abstention interval concept, to drop from the database ratings that do not reflect the current social network user{\textquoteright}s interests, thus improving prediction quality.}, keywords = {collaborative filtering, concept drift, Evaluation, Recommender Systems, shift of interest, Social Networks}, doi = {10.3390/informatics5020021}, url = {http://www.mdpi.com/2227-9709/5/2/21}, author = {Dionisis Margaris and Costas Vassilakis} } @article {margaris-tldks, title = {Enhancing User Rating Database Consistency Through Pruning}, journal = {Transactions on Large-Scale Data- and Knowledge-Centered Systems XXXIV}, volume = {LNCS, volume 10620}, number = {Special issue on Consistency and Inconsistency in Data-centric Applications}, year = {2017}, pages = {33{\textendash}64}, publisher = {Springer Berlin Heidelberg}, abstract = {Recommender systems are based on information about users{\textquoteright} past behavior to formulate recommendations about their future actions. However, as time goes by the interests and likings of people may change: people listen to different singers or even different types of music, watch different types of movies, read different types of books and so on. Due to this type of changes, an amount of inconsistency is introduced in the database since a portion of it does not reflect the current preferences of the user, which is its intended purpose. In this paper, we present a pruning technique that removes old aged user behavior data from the ratings database, which are bound to correspond to invalidated preferences of the user. Through pruning (1) inconsistencies are removed and data quality is upgraded, (2) better rating prediction generation times are achieved and (3) the ratings database size is reduced. We also propose an algorithm for determining the amount of pruning that should be performed, allowing the tuning and operation of the pruning algorithm in an unsupervised fashion. The proposed technique is evaluated and compared against seven aging algorithms, which reduce the importance of aged ratings, and a state-of-the-art pruning algorithm, using datasets with varying characteristics. It is also validated using two distinct rating prediction computation strategies, namely collaborative filtering and matrix factorization. The proposed technique needs no extra information concerning the items{\textquoteright} characteristics (e.g. categories that they belong to or attributes{\textquoteright} values), can be used in all rating databases that include a timestamp and has been proved to be effective in any size of users-items database and under two rating prediction computation strategies. Note: This material is presented to ensure timely dissemination of scholarly and technical work. Copyright and all rights therein are retained by authors or by other copyright holders. All persons copying this information are expected to adhere to the terms and constraints invoked by each author{\textquoteright}s copyright. In most cases, these works may not be reposted without the explicit permission of the copyright holder.}, keywords = {Collaborative filtering, Evaluation, Internet of Things, performance, Recommendation Applications, Social Networks, Web services}, doi = {10.1007/978-3-662-55947-5_3}, url = {https://link.springer.com/article/10.1007\%2Fs11761-017-0216-y}, author = {Dionisis Margaris and Costas Vassilakis} } @conference {antoniouEuromed2017, title = {Experiences from the development of thematic itineraries in three Greek museums (Greek title: Εμπειρίες από την ανάπτυξη θεματικών διαδρομών σε τρία Ελληνικά μουσεία)}, booktitle = {Proceedings of the 2nd PanHellenic conference on Cultural Heritage (Euromed 2017)}, year = {2017}, month = {12}, abstract = {Private museums, as well as a considerable number of public regional museums, are confined by the small exhibition space and the relatively small number of exhibits and therefore cannot develop multiple exhibitions to satisfy visitors with different interests and characteristics. In this paper we report on our experieces from developing multiple thematic itineraries in the Tegea Folklore Museum , in the Cretan open-air museum "Lychnostatis" in Heraclion, and in the Panarcadic Archaeological Museum of Tripolis. The thematic itineraries in the Tegea Folklore Museum are (a) space (b) technology and (c) clothing. In the Cretan open-air museum "Lychnostatis", the itineraries are (a) natural sciences,(b) popular art, (c) pre-industrial technology and (d) folklore. In the Panarcadic Archaeological Museum of Tripolis we have the following itineraries: (a) appearance/clothing, (b) mortality, (c) nudity, (d) religion, (e) social status, (f) education, (g) healing practices and (h) everyday life. Each itinerary exploits a subset of the available exhibits to present a selected subject and is adapted to the needs of a predetermined and focused group of the general public. The results from the application of the approach, insofar, have shown that this is a promising path, both regarding the better exploitation of the space and the exhibits of the small museums, as well as regarding the better adaptation to the interests of the visitors.}, author = {Angela Antoniou and Costas Vassilakis and Manolis Wallace and Giorgos Lepouras and Vassilis Poulopoulos} } @article {margaris-soca-17, title = {Exploiting Internet of Things information to enhance venues{\textquoteright} recommendation accuracy}, journal = {Service Oriented Computing and Applications}, volume = {todb}, year = {2017}, month = {aug}, pages = {393{\textendash}409}, publisher = {Springer Nature}, abstract = {In this paper, we introduce a novel recommendation algorithm, which exploits data sourced from web services provided by the Internet of Things in order to produce more accurate venue recommendations. The proposed algorithm provides added value for the web services offered by the Internet of Things and enhances the state-of-the-art in this algorithm category by taking into account (a) web of things data regarding the contexts of the user and the context of the venues to be recommended (restaurants, movie theatres, etc.), such as the user{\textquoteright}s geographical position, road traffic and weather conditions, (b) qualitative aspects of the venues, such as price, atmosphere or service, (c) the semantic similarity of venues and (d) the influencing factors between social network users, derived from user participation in social networks. The combination of these features leads to more accurate and better user-targeted recommendations. We also present a framework which incorporates the above characteristics, and we evaluate the presented algorithm, both in terms of performance and recommendation quality.}, keywords = {collaborative filtering, Evaluation, Internet of Things, performance, Recommendation Applications, Social Networks, Web services}, doi = {10.1007/s11761-017-0216-y}, url = {https://link.springer.com/article/10.1007/s11761-017-0216-y}, author = {Dionisis Margaris and Costas Vassilakis} } @article {katifori15a, title = {Effectiveness of visualization for information retrieval through ontologies with entity evolution: the impact of ontology modeling}, journal = {International Journal of Information Retrieval Research}, volume = {5}, number = {2}, year = {2015}, pages = {26}, abstract = {Incorporating digital tools in the business and scientific research workflows is at the moment an on-going process, challenging and demanding as every domain has its own needs in terms of data models and information retrieval methods. The information in some domains involves entity evolution, a characteristic that introduces additional tasks, such as finding all evolution stages of an entity, and poses additional requirements for the information retrieval process. In this paper, we present a user study aiming to investigate how the different aspects of ontology modelling affect the performance and effectiveness of users regarding information retrieval tasks that are carried out using visualization methods. The results of the user study are analyzed and guidelines for ontology design are offered.}, doi = {10.4018/IJIRR.2015040104}, author = {Akrivi Katifori and Costas Vassilakis and Giorgos Lepouras and Elena Torou} } @inbook {benou2009a, title = {Encyclopedia of Information Science and Technology, Second Edition}, year = {2009}, pages = {1-7}, publisher = {IGI Global}, organization = {IGI Global}, chapter = {Exploiting Context in Mobile Applications}, abstract = {In the mobile and pervasive computing environment, software engineering should not treat diversity and mobility as problems to overcome, but seek methods of which it could take advantage instead. In these environments, the selection of purpose-oriented and timely information, tailored to user preferences and media characteristics will ensure an optimised information delivery. To this end, the context .the information that surrounds the human-computer interaction. plays a key role and is rapidly changing in mobile settings, and the understanding of it is indispensable for the application designers in order to choose, capture and exploit it. In this work, we present an architecture facilitating the collection of context information and its exploitation for optimized information delivery, within mobile and pervasive computing environments.}, doi = {10.4018/978-1-60566-026-4.ch236}, url = {https://www.igi-global.com/chapter/exploiting-context-mobile-applications/13774}, author = {Poulcheria Benou and Costas Vassilakis}, editor = {Mehdi Khosrow-Pour} } @conference {vassiliadis2008d, title = {Evaluation Study of a Wireless Multimedia Traffic-Oriented Network Model}, booktitle = {INTERNATIONAL ELECTRONIC CONFERENCE ON COMPUTER SCIENCE. AIP Conference Proceedings, Volume 1060}, year = {2008}, month = {7}, pages = {379-384}, abstract = {In this paper, a wireless multimedia traffic-oriented network scheme over a fourth generation system (4-G) is presented and analyzed. We conducted an extensive evaluation study for various mobility configurations in order to incorporate the behavior of the IEEE 802.11b standard over a test-bed wireless multimedia network model. In this context, the Quality of Services (QoS) over this network is vital for providing a reliable high-bandwidth platform for data-intensive sources like video streaming. Therefore, the main issues concerned in terms of QoS were the metrics for bandwidth of both dropped and lost packets and their mean packet delay under various traffic conditions. Finally, we used a generic distance-vector routing protocol which was based on an implementation of Distributed Bellman-Ford algorithm. The performance of the test-bed network model has been evaluated by using the simulation environment of NS-2.}, doi = {10.1063/1.3037098}, author = {D.C. Vasiliadis and G.E. Rizos and Costas Vassilakis} } @conference {Christos2008248, title = {Exception resolution for BPEL processes: A middleware-based framework and performance evaluation}, booktitle = {Proceedings of the 10th International Conference on Information Integration and Web-based Applications and Services, iiWAS 2008}, year = {2008}, note = {cited By (since 1996) 0}, month = {12}, pages = {248-256}, abstract = {WS-BPEL is widely used nowadays for specifying and executing composite business processes within the Service Oriented Architecture (SOA). During the execution however, of such business processes, a number of faults stemming from the nature of SOA (e.g. network or server failures) may occur. The WS-BPEL scenario designer must therefore use the provisions offered by WS-BPEL to catch these exceptions and resolve them, usually by invoking some equivalent web service that is expected to be reachable and available. System fault handler specification is though an additional task for the WS scenario designer, while the presence of such handlers within the scenario necessitates extra maintenance activities, as new alternate services emerge or some of the specified ones are withdrawn. In this paper, we propose a middleware-based framework for system exception resolution, which undertakes the tasks of failure interception, discovery of alternate services and their invocation. The middleware is deployed and maintained independently of the WS-BPEL scenarios, removing thus the need for specifying and maintaining system fault handlers within the scenarios. We also present performance measures, establishing that the overhead imposed by the addition of the proposed middleware layer is minimal. Β{\textcopyright} 2008 ACM.}, keywords = {Ad hoc networks, Business Process, Exception handling, Information retrieval, Information services, Maintenance activity, Middleware, Middleware layer, Performance evaluation, Performance measure, Performance metrics, Quality control, Quality of service, Quality of service (QoS), Scalability, Service oriented architecture (SOA), System faults, Telecommunication services, Web services, WS-BPEL}, doi = {10.1145/1497308.1497355}, url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-70349146138\&partnerID=40\&md5=9a5231fe52fffef11707efd4f3489a37}, author = {Christos Kareliotis and Costas Vassilakis and S. Rouvas and P. Georgiadis} } @inbook {lep2007a, title = {Encyclopedia of Digital Government}, year = {2007}, pages = {1413{\textendash}1417}, publisher = {IGI Publishing}, organization = {IGI Publishing}, chapter = {Reusability In Governmental Electronic Services}, abstract = {Reusability is the degree to which a software component or other work can be used in more than one programs or software systems. e-Government is a prosperous area for the application of reusability, since the services offered to citizens from the same administration, or even different administrations, have common portions that can be developed only once and reused wherever appropriate. In this paper we present the design and implementation of an electronic service development environment which offers the potential to reuse components that have already been implemented, for the realization of new services.}, doi = {10.4018/978-1-59140-789-8.ch216}, author = {Giorgos Lepouras and Costas Vassilakis}, editor = {Ari-Veikko Anttiroiko and Matti Malkia} } @inbook {vas2007a, title = {Encyclopedia of Digital Government}, year = {2007}, pages = {1174{\textendash}1179}, publisher = {IGI Publishing}, organization = {IGI Publishing}, chapter = {The Lifecycle Of Transactional Services}, abstract = {Transactional services are an indispensible part of e-government, since provision of services to citizens and enterprises,as well as the interaction between the government and citizens-enterprises are modelled mainly through such services. Latest quantifications, however, show that the development of such services lags behind as compared both to the expectations of citizens-enterprises and to the efforts made by governments. This can be attributed, amongst other reasons, to the "traditional" approach to electronic service development, which treats each electronic service as an isolated software project. In this paper, we propose an e-service development platform, which covers the whole lifecycle of transactional services and facilitates the analysis, development, deployment and maintenance of these services.}, doi = {10.4018/978-1-59140-789-8.ch177}, author = {Costas Vassilakis and Giorgos Lepouras}, editor = {Ari-Veikko Anttiroiko and Matti Malkia} } @conference {DBLP:conf/icws/KareliotisVG07, title = {Enhancing BPEL scenarios with Dynamic Relevance-Based Exception Handling}, booktitle = {ICWS}, year = {2007}, month = {7}, pages = {751-758}, abstract = {Web services have become the key technology in business processes management. Business processes can be self-contained or be composed from sub-processes; the latter category is typically specified using the Web Services Business Process Execution Language (WS-BPEL) and executed by a Web Services Orchestrator (WSO). During the execution however of such a composite service, a number of faults stemming from the distributed nature of the SOA architecture, e.g. network or server failures may occur. WS-BPEL includes provisions for exception handling, which can be exploited for detecting such failures; once detected, a failure can be resolved by invoking alternate web service implementations that perform the same business task as the failed one. However, the inclusion of such provisions is a tedious assignment for the business process designer, while additional effort would be required to maintain the BPEL scenarios in cases that some alternate WS implementations cease to exist or new ones are introduced. In our research we are developing a framework for automating handling of that kind of exceptions. The proposed solution employs a pre-processor that enhances BPEL scenarios with code that detects failures, discovers alternate WS implementations and invokes them, fully thus resolving the exception. Alternate WS implementation discovery is based on service relevance, which takes into account both functional and qualitative properties of web services.}, doi = {10.1109/ICWS.2007.86}, author = {Christos Kareliotis and Costas Vassilakis and P. Georgiadis} } @conference {Golemati200793, title = {Evaluating the significance of the windows explorer visualization in personal information management browsing tasks}, booktitle = {Proceedings of the International Conference on Information Visualisation}, year = {2007}, note = {cited By (since 1996) 0}, month = {7}, pages = {93-98}, abstract = {The visualization of hierarchies is very important for digital information management and presentation systems. Especially in the context of Personal Information Management, file browsers play a very important role. Currently the most common file browser visualizations are Windows Explorer and the simple zoomable visualization offered by Microsoft Windows. This work explores the issue of file browser visualization through a user study based on inter-views and an experiment. Β{\textcopyright} 2007 IEEE.}, keywords = {Data reduction, File browser visualization, Hierarchical systems, Indented list visualization, Information analysis, User study, Web browsers, Windows explorer}, doi = {10.1109/IV.2007.46}, url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-35348919670\&partnerID=40\&md5=f35f306d4063ee2e91e3c573f4ed5d8a}, author = {M. Golemati and Akrivi Katifori and E.G. Giannopoulou and I. Daradimos and Costas Vassilakis} } @inbook {vas2006a, title = {Encyclopedia of E-Commerce, E-Government and Mobile Commerce}, year = {2006}, pages = {865{\textendash}870}, publisher = {Idea Group Inc.}, organization = {Idea Group Inc.}, chapter = {Ontology for e-Government public services}, abstract = {In this work, the usage of ontologies for meeting requirements related to e-service composition, e-service cataloguing, change management and administrative responsibility is examined. An ontology for e-government services is presented, covering various aspects of services, including administrative responsibility, meta-data, involved documents and legislation. Both the development and usage phase of the ontology are covered and directions for further exploitation of the potential offered by the ontological representation are given.}, doi = {10.4018/978-1-59140-799-7.ch139}, author = {Costas Vassilakis and Giorgos Lepouras}, editor = {Mehdi Khosrow-Pour} } @inbook {lep2006a, title = {Encyclopedia of E-Commerce, E-Government and Mobile Commerce}, year = {2006}, pages = {1064{\textendash}1069}, publisher = {Idea Group Inc.}, organization = {Idea Group Inc.}, chapter = {Tailorable e-government information systems}, abstract = {Real-world information, knowledge and procedures after which information systems are modeled are generally of dynamic nature and subject to changes, due to the emergence of new requirements or revisions to initial specifications. E-government information systems (eGIS) present a higher degree of volatility in their environment, since requirement changes may stem from multiple sources, including legislation changes, organizational reforms, end-user needs, interoperability and distribution concerns etc. To this end, the design and implementation of eGIS must adhere to paradigms and practices that facilitate the accommodation of changes to the eGIS as they occur in the real world. In this work, we present a role-based model for designing and implementing eGIS that can dynamically accomodate changes, providing the necessary facilities for modeling multiple aspects of the same real-world entities and delivering context-specific behaviour.}, doi = {10.4018/978-1-59140-799-7.ch171}, author = {Giorgos Lepouras and Anya Sotiropoulou and Dimitrios Theotokis and Costas Vassilakis}, editor = {Mehdi Khosrow-Pour} } @inbook {lep2006b, title = {Encyclopedia of E-Commerce, E-Government and Mobile Commerce}, year = {2006}, pages = {1{\textendash}6}, publisher = {Idea Group Inc.}, organization = {Idea Group Inc.}, chapter = {Adaptive Virtual Reality Shopping Malls}, abstract = {Firms and organizations are increasingly exploiting electronic channels to reach their customers and create new business opportunities. To this end, electronic shops have been developed, either offering products from a single firm or encompassing multiple individual electronic stores, comprising thus electronic shopping malls. Two main concerns for e-commerce are personalization and enhancement of user experience. Personalization addresses the ability to offer content tailored to the preferences of each user or user group. Preferences may be explicitly declared by the user, or derived by the system through inspecting user interaction; if the system dynamically reacts to changes of visitor behavior, it is termed as adaptive. Enhancement of user experience is another major issue in e-commerce, given that 2D-images and texts on the screen are not sufficient to provide information on products aspects such as physical dimensions, textures and manipulation feedback. Multimedia presentations can also be used as a means for .information acceleration. for promoting "really new" products. This article aims to specify a system that exploits capabilities offered by adaptation and VR technologies to offer e-shoppers personalized and enhanced experiences, while addressing challenges related to the cost, complexity and effort of building and maintaining such a system.}, doi = {10.4018/978-1-59904-955-7.ch099}, author = {Giorgos Lepouras and Costas Vassilakis}, editor = {Mehdi Khosrow-Pour} } @article {Vassilakis200581, title = {Exploiting form semantics and validation checks to improve e-form layout}, journal = {International Journal of Web Engineering and Technology}, volume = {2}, number = {1}, year = {2005}, note = {cited By (since 1996) 0}, pages = {81-96}, abstract = {On their route to the electronic era, organisations release on the web more and more complex form-based services, which require users to enter numerous data items interrelated by business rules. In such a context, it is crucial to provide optimal form layouts, in order to present the service users with interfaces that facilitate their work. This paper presents an integrated environment, which exploits data item interrelations manifested by the business rules (or validation checks), to optimise the layout of the web forms comprising a complex service. The approach is validated through its application on a tax return form e-service. Copyright Β{\textcopyright} 2005 Inderscience Enterprises Ltd.}, keywords = {electronic services, Form layout, Form semantics, Integrated environment, Interfaces (computer), Optimization, semantics, Taxation, Usuability, Validation checks, World Wide Web}, doi = {10.1504/IJWET.2005.007465}, url = {http://www.scopus.com/inward/record.url?eid=2-s2.0-24344507764\&partnerID=40\&md5=e94b487785303410cb7bf88df9d12c86}, author = {Costas Vassilakis and Giorgos Lepouras and S. Rouvas and P. Georgiadis} } @conference {gorilas2003, title = {E-forms services for the Public Sector: Shifting Development Effort from Programmers to Domain Experts}, booktitle = {Proceedings of e-Challenges 2003}, year = {2003}, month = {10}, abstract = {E-forms have a central role in a significant number of e-government services. This paper presents a knowledge-based technical platform aiming to assist public sector employees to generate online transaction services by simplifying their development, maintenance and integration with installed IT systems. At the heart of this platform lies the knowledge and transaction services repository. This repository consists of a number of XML document types that incorporate all necessary details for creating and managing online transaction services. The main underlying idea is to provide a platform with intuitive interfaces that can be used directly by domain experts thus minimising the need for personnel with IT skills. This platform is currently under development within the IST SmartGov project.}, author = {Stelios Gorilas and Costas Vassilakis and Tomas Pariente Lobo and E. Tambouris} } @conference {Lask2001a, title = {e-Citizens and e-Consumers: Is There a Difference?}, booktitle = {Proceedings of the PCHCI 2001 Workshop: "Transforming Web Surfers to E-Shoppers"}, year = {2001}, month = {12}, pages = {47-50}, abstract = {Both the business sector and the government are nowadays embracing Internet technologies in order to provide high quality on-line services to their "target groups". In both cases, service providers are trying to transform web surfers, casually visiting their web sites to seek information, to users of their electronic services, i.e. e-consumers and. e-citizens. In this paper, we address the similarities and the differences between the business and the government, when they act as service providers, with respect to the factors for successful service and the issues that must be addressed.}, author = {Laskaridis, G. and Costas Vassilakis and Giorgos Lepouras and S. Rouvas} } @conference {1993-boukouvalas-hci, title = {An Enhanced System for File Access in a Distributed UNIX Environment}, booktitle = {Proceedings of the 4th Hellenic Conference on Informatics}, year = {1993}, month = {12}, abstract = {In this study we present an Enhanced File System for a Distributed Unix Environment. The Enhanced Unix File System implements a flexible protection mechanism for files and directories. It is based on the concept of Access Control Lists (ACL), which allows different permissions for files and directories to be given to specific users. His work was developed under the SunOS/NFS distributed environment, using Remote Procedure Calls (RPC) to implement the communication between the client and the server. The system consists of a daemon process working as a server and a set of client processes that provide different file and directory services. There may be running many different servers and clients over the same network. The client processes transparently locate the appropriate server for each transaction. The system also provides an open programming environment for the development of new applications. Both the user and the programmer interface of the system are kept close to standard UNIX.}, url = {http://cst.uop.gr/images/papers/1993-kassapidis-hci.pdf}, author = {Boucouvalas, C. and Costas Vassilakis and P. Georgiadis} }