Graphical lasso for covariance structure learning in the high dimensional setting

Abstract: This thesis considers the estimation of undirected Gaussian graphical models especially in the high dimensional setting where the true observations are assumed to be non-Gaussian distributed. The first aim is to present and compare the performances of existing Gaussian graphical model estimation methods. Furthermore since the models rely heavily on the normality assumption, various methods for relaxing the normal assumption are presented. In addition to the existing methods, a modified version of the joint graphical lasso method is introduced which monetizes on the strengths of the community Bayes method. The community Bayes method is used to partition the features (or variables) of datasets consisting of several classes into several communities which are estimated to be mutually independent within each class which allows the calculations when performing the joint graphical lasso method, to be split into several smaller parts. The method is also inspired by the cluster graphical lasso and is applicable to both Gaussian and non-Gaussian data, assuming that the normal assumption is relaxed. Results show that the introduced cluster joint graphical lasso method outperforms com-peting methods, producing graphical models which are easier to comprehend due to the added information obtained from the clustering step of the method. The cluster joint graphical lasso is applied to a real dataset consisting of p = 12582 features which resulted in computation gain of a factor 35 when comparing to the competing method which is very significant when analysing large datasets. The method also allows for parallelization where computations can be spread across several computers greatly increasing the computational efficiency.