Branches were produced by agglomerative clustering in R using cor() and hclust(), and the resulting tree cut to highlight major divisions.
Seven topics are excluded from this figure because they were especially non-content-bearing, e.g. topics comprised of non-words formed by unsuccessful character recognition during the scanning process or topics indicating merely a non-English language.
To be linked in this figure, two topics must each contribute at least 12 percent to the same dissertation, and such a pairing must occur at least 4 times.