When I run a query for "hers" in Google Ngram Viewer, I get back the word's frequency of occurrence as a percentage. We know the outcome percentage; what's the denominator on the other size? Is it 100 million words? More?
Michel, Jean-Baptiste, et al. "Quantitative analysis of culture using millions of digitized books." science 331.6014 (2011): 176-182. is the publication that describes the data set:
The resulting corpus contains over 500 billion words, in English (361 billion), French (45B), Spanish (45B), German (37B), Chinese (13B), Russian (35B), and Hebrew (2B). The oldest works were published in the 1500s. The early decades are represented by only a few books per year, comprising several hundred thousand words. By 1800, the corpus grows to 60 million words per year; by 1900, 1.4 billion; and by 2000, 8 billion.