Drinking to point of vomiting

You may, however, notice that the Spearman similarities for individual years on this graph are about .1 lower than they were when we graphed fiction as a 39-year moving window. In principle Spearman similarity is independent of corpus size, but it can be affected by the diversity of a corpus. The similarity between two individual texts is generally going to be lower than the similarity between two large and diverse corpora. So could the changes we’ve seen be produced by changes in corpus size? There could be some effect, but I don’t think it’s large enough to explain the phenomenon. [See update at the bottom of this post. The results are in fact even clearer when you keep corpus size constant. -Ed.] The sizes of the corpora for different genres don’t change in a way that would produce the observed decreases in similarity; the fiction corpus, in particular, gets larger as it gets less like nonfiction. Meanwhile, it is at the same time becoming more like poetry. We’re dealing with some factor beyond corpus size.

Scour DH literature all you want, but you will find no head-on treatment of the theoretical problems that such methods entail.