Block-diagonal precision matrix regularization for ultrahigh dimensional data

A method that estimates the precision matrix of multiple variables in the extreme scope of “ultrahigh dimension” and “small sample-size” is proposed. Initially, a covariance column-wise screening method is provided in order to identify a small sub-group, which are significantly correlated, from thousands and even millions of variables. Then, a regularization of block-diagonal covariance structure of the thousands or millions of variables is imposed, in which only the covariances of variables in that small sub-group are retained and all others vanish. It is further proven that under some mild conditions the vital sub-group identified by the covariance column-wise screening method is consistent. A major advantage of the proposed method is its efficiency - it produces a reliable precision matrix estimator for thousands of variables within a few of seconds while the existing methods take at least several hours and even so still yield inaccurate estimators. Empirical data studies and numerical simulations show that the proposed precision matrix estimation greatly outperforms existing methods in the sense of taking much less computing time and resulting in much more accurate estimation when dealing with ultrahigh dimensional data.