When dealing with the HathiTrust public domain volumes, the makeup of the collection changes drastically. This is because US copyright law makes it difficult to ascertain rights post-1923, resulting in representation biases. There are proportionally less public domain works approaching the present day, but the drop-off is much quicker in some categories than others. For a valid understanding of the collection, one shouldn't compare pre-1923 and 1923- subsets of the public domain collection without additional context.
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