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.
Figure: Proportion (in %) of collection represented by each LC Class, by Year. 1923 is marked with a vertical line.
Table: Library of Congress class distributions for Public Domain data, in % of Collection
| Library of Congress Class | pre-1923 | 1923- | Difference |
|---|---|---|---|
| Language and Literature | 9.58% | 1.84% | -7.74% |
| General and Old World History | 6.02% | 1.63% | -4.39% |
| Science | 4.04% | 4.31% | 0.27% |
| Philosophy, Psychology, and Religion | 4.01% | 1.06% | -2.95% |
| Social Sciences | 3.53% | 12.01% | 8.47% |
| General Works | 2.74% | 0.25% | -2.49% |
| History of the United States and British, Dutch, French, and Latin America | 2.53% | 1.06% | -1.47% |
| Law | 2.34% | 5.67% | 3.33% |
| Technology | 2.06% | 6.47% | 4.41% |
| History of America | 1.75% | 1.30% | -0.45% |
| Political Science | 1.70% | 2.45% | 0.75% |
| Agriculture | 1.48% | 3.93% | 2.45% |
| Education | 1.22% | 1.79% | 0.57% |
| Medicine | 1.06% | 2.12% | 1.06% |
| Bibliography, Library Science, and General Information Resources | 1.01% | 2.63% | 1.62% |
| Fine Arts | 0.97% | 0.65% | -0.33% |
| Music | 0.87% | 0.46% | -0.41% |
| Geography, Anthropology, and Recreation | 0.77% | 1.38% | 0.61% |
| Auxiliary Sciences of History | 0.54% | 0.81% | 0.27% |
| Military Science | 0.31% | 1.49% | 1.18% |
| Naval Science | 0.16% | 0.59% | 0.42% |
(HT PD-only, Spring 2015)