The Statistics Seminar speaker for Wednesday, May 1, 2019, is Eugen Pircalabelu, a Lecturer (Chargé de cours) at UC Louvain (Belgium) working at the Institute of Statistics, Biostatistics and Actuarial Sciences within LIDAM at the Faculty of Science. He received his PhD in Statistics from KU Leuven in 2015. Prior to moving at UC Louvain, he held a Visiting professor position at Ghent University affiliated with the Department of Applied Mathematics, Computer Science and Statistics and a Postdoctoral position at KU Leuven affiliated with the Research Centre for Operations Research and Business Statistics (ORSTAT). Professor Pircalabelu's research interests focus on model selection and estimation for graphical models and high-dimensional statistics with applications to fMRI brain imaging problems. His research papers have appeared in journals such as Annals of Applied Statistics, Biostatistics, Statistics and Computing and Social Networks.

**Talk: **Community detection on probabilistic graphical models with group-based penalties

**Abstract: **A new strategy of probabilistic graphical modeling is developed that draws parallels from social network analysis. Probabilistic graphical modeling summarizes the information coming from multivariate data in a graphical format where nodes, corresponding to random variables, are linked by edges that indicate dependence relations between the nodes. The purpose is to estimate the structure of the graph (which nodes connect to which other nodes) when data at the nodes are available. On the opposite side of the spectrum, social network analysis considers the graph as the observed data. Given thus the graph where connections between nodes are observed rather than estimated, social network analysis estimates models that represent well an underlying mechanism which has generated the observed graph.

We propose a new method that exploits the strong points of each framework as it estimates jointly an undirected graph and communities of homogenous nodes, such that the structure of the communities is taken into account when estimating the graph and conversely, the structure of the graph is accounted for when estimating homogeneous communities of nodes. The procedure uses a joint group graphical lasso approach with community detection-based grouping, such that some groups of edges co-occur in the estimated graph. The grouping structure is unknown and is estimated based on community detection algorithms.

Theoretical derivations regarding graph convergence and sparsistency, as well as accuracy of community recovery are included, while the method’s empirical performance is illustrated in an fMRI context, as well as with simulated examples. Joint work with Gerda Claeskens.