1. A computer-implemented method for categorizing digital documents, containing digital content, in aggregate, the method performed by a computer processor and comprising: (a) receiving by the computer processor a training set of digital documents each containing digital content, each digital document in the training set being assigned to one of a plurality of categories and being associated with one of a plurality of content profiles, each content profile representing existence or absence of one or more features in the digital content of the digital document; (b) receiving by the computer processor a population set of digital documents each containing digital content, each digital document in the population set having one of the plurality of content profiles associated with the digital content contained therein; (c) organizing the digital documents of the training set and the digital documents of the population set into a matrix using the plurality of content profiles, the matrix having rows corresponding to each of the digital documents and cells indicating existence or absence of the one or more features in the digital content of the digital document; (d) determining a weight for each row of the matrix using an estimated total variance for that row of the matrix; (e) determining, by the computer processor applying a stacked regression coupled with weighted regression to the matrix, the weighted regression using the weights determined for the rows of the matrix, a proportion of the digital documents in the population set belonging to each category of the plurality of categories; (f) determining one or more category proportions of the digital documents, each including the portion of the digital documents belonging each category; and (g) categorizing the digital documents by labeling the digital document based on the category corresponding to the proportion to which the digital document belongs.