Ofertes de projectes

In greater detail, the two directions consist of 

(1) Providing an efficient implementation of the optimization algorithm. See Ferrer-i-Cancho and Solé (2003) and Ferrer-i-Cancho (2005) for further details about the algorithm. Evaluating the cost for a given bipartite graph from scratch has cost of the order of nm, where n is the number of words and m is the number of meanings. Decinding when to stop the optimizacion algorithm requires (nm)^2 evaluations of the cost function (in practice it had to be cut down to about nm due to computational costs). For these reasons, n and m have been kept small in previous studies compared to real values in fully fledged human language (e.g., n = m = 150 in Ferrer-i-Cancho and Solé 2003). This computational callenge would be solved applying different techniques, e.g., (a) parallelization (b) dynamic calculation (when changing a few cells of the adjacency matrix, the cost function should not be computed from scratch) and (c) heuristics to speed up the Monte Carlo scheme.

(2) Comparing the statistical properties of the model against the real statistical properties of human language (e.g., linguistics laws) and animal communication, including properties that have not been tested in previous research on these models. See Ferrer-i-Cancho (2018) for an overview of some of the statistical properties of real language that could be tested.

Depending on the personal interests of the student, the project can focus in one of the two directions.

It is possible to publish the results of the project in a research journal. 

REFERENCES

Ferrer-i-Cancho, R. & Solé, R. V. (2003). Least effort and the origins of scaling in human language. Proceedings of the National Academy of Sciences USA 100, 788-791. 

Ferrer-i-Cancho, R. (2005). Zipf's law from a communicative phase transition. European Physical Journal B 47, 449-457.

Ferrer-i-Cancho, R. (2017). The optimality of attaching unlinked labels to unlinked meanings. Glottometrics 36, 1-16. 

Ferrer-i-Cancho, R. & Vitevitch, M. S. (2018). The origins of Zipf's meaning-frequency law. Journal of the American Society for Information Science and Technology 69 (11), 1369-1379.

Ferrer-i-Cancho, R. (2018). Optimization models of natural communication. Journal of Quantitative Linguistics 25 (3), 207-237.

Zipf, G.K. (1949). Human behaviour and the principle of least effort. Cambridge (MA), USA: Addison-Wesley. 

Most of the research on group recommendation investigated the core algorithms used for recommendation generation.  Two different strategies have been mostly used for generating group recommendations: aggregating individual predictions into group predictions or aggregating individual models into group models. Differences among these strategies differ in the timing of data aggregation step. 

In fact, the role of a group recommender system is to make suggestions that reflect the preferences of the group as a whole, while offering reasonable and acceptable options to individual group members. An important issue to be addressed in this kind of recommenders is how to reach consensus among members during and at the end of the recommendation process.

In this proposal, the focus will be on generating a group recommender framework that will be based on the well-known LibRec library. The main steps will be: 

1. Adapt Librec to be a GroupLibRec library 
2. Include in GroupLibRec the standard methods for generating group recommendations 
3. Propose new methods for generating group recommendations 
4. Analyze the proposals against well-known state-of-the-art methods

Boltzmann Machines are probabilistic models developed in 1985 by
D.H. Ackley, G.E. Hinton and T.J. Sejnowski. In 2006, Restricted
Boltzmann Machines (RBMs) were used in the pre-training step of
several successful deep learning models, leading to a new renaissance
of neural networks and artificial intelligence.

In spite of their nice mathematical formulation, there are a number of
issues that are hard to compute:

1. The computation of the partition function is NP-hard, involving an  exponential sum of terms
2. The exact computation of the derivative of the log-likelihood is  also NP-hard, since it contains the derivative of the partition function

Therefore, in practice we have to approximate both the computation of
the probabilities and several components of the learning process
itself. These drawbacks have prevented RBMs to show their real
potential as truly probabilistic models.

Currently, we are working on trying to improve several of the unsolved
issues related to RBMs:

1. - Mechanisms to control the learning process: www.lsi.upc.edu/~eromero/Publications/Downloads/2018-tnnls-stopcritRBM.pdf
2. Better approximations of the derivative of the log-likelihood: http://www.lsi.upc.edu/~eromero/Publications/Downloads/2019-nn-weightedCD.pdf
3. Efficient approximation of the partition function (work in progress)

These works have opened new lines of research, some of which can be
the topic of a Master's Thesis. The scope and degree of depth of the
work can be adapted to the estimated times to complete the Thesis. For
further details, contact Enrique Romero (eromero@cs.upc.edu).

A more detailed description of the project can be found in

https://www.cs.upc.edu/~eromero/Downloads/Retina-TFM-Project-01.pdf

The project is proposed in collaboration with Javier Zarranz Ventura
(Institut Clínic d'Oftalmologia, ICOF, Hospital Clínic de Barcelona, and
Institut d'Investigacions Biomèdiques August Pi I Sunyer, IDIBAPS),
which would provide a large annotated database to develop the project. For further information, please contact Alfredo Vellido (avellido@cs.
upc.edu) or Enrique Romero (eromero@cs.upc.edu).

A detailed description of the project can be found in the following link:

https://drive.google.com/open?id=1jTP8-kK0j3ySa5_LH7OQ-BeLLenGCt7G

Recommender systems analyze the behavior of the users and their preferences, to learn patterns and understand what might be interesting to suggest them[4]. Natural imbalances in the data (e.g., in the amount of observations for a subset of popular items) might be embedded in the patterns. As a consequence, the produced recommendations exacerbate these imbalances, thus strengthening inequalities and generating biases [2]. When these imbalances are associated to sensitive attributes of the users (e.g., gender or race), this might have negative societal consequences, such as unfairness [3]. Unfairness might affect all the stakeholders involved in a recommender system, such as the users (when the minority receives systematically worse recommendations), or the content providers (when the items offered by a group of providers are exposed less than those of their counterpart) [1].

The goal of this project is to analyze and mitigate bias in recommender systems. To do it, we will consider state-of-the-art recommender systems, covering both model- and memory -based approaches and point-wise and pair-wise algorithms. 

[1] Himan Abdollahpouri, Gediminas Adomavicius, Robin Burke, Ido Guy, Dietmar Jannach, Toshihiro Kamishima, Jan Krasnodebski, and Luiz AugustoPizzato. 2020. Multistakeholder recommendation: Survey and research directions.User Model. User-Adapt. Interact.30, 1 (2020), 127¿158. https://doi.org/10.1007/s11257-019-09256-1

[2] Ludovico Boratto, Gianni Fenu, and Mirko Marras. 2019. The Effect of Algorithmic Bias on Recommender Systems for Massive Open Online Courses.InAdvances in Information Retrieval - 41st European Conference on IR Research, ECIR 2019, Cologne, Germany, April 14-18, 2019, Proceedings, Part I(Lecture Notes in Computer Science, Vol. 11437), Leif Azzopardi, Benno Stein, Norbert Fuhr, Philipp Mayr, Claudia Hauff, and Djoerd Hiemstra (Eds.).Springer, 457¿472. https://doi.org/10.1007/978-3-030-15712-8_30

[3] Sara Hajian, Francesco Bonchi, and Carlos Castillo. 2016. Algorithmic Bias: From Discrimination Discovery to Fairness-aware Data Mining.InProceedings of the 22nd ACM SIGKDD International Conference on Knowledge Discovery and Data Mining, San Francisco, CA, USA, August 13-17, 2016, Balaji Krishnapuram, Mohak Shah, Alexander J. Smola, Charu C. Aggarwal, Dou Shen, and Rajeev Rastogi (Eds.). ACM, 2125¿2126.https://doi.org/10.1145/2939672.2945386

[4] Francesco Ricci, Lior Rokach, and Bracha Shapira. 2015. Recommender Systems: Introduction and Challenges. InRecommender Systems Handbook,Francesco Ricci, Lior Rokach, and Bracha Shapira (Eds.). Springer, 1¿34. https://doi.org/10.1007/978-1-4899-7637-6_