A. By counting reproductions in art text books.
David Galenson, a professor in the Department of Economics at the University of Chicago, has brought quantitative research methods to bear on questions like "how do we decide which artists are 'great'?" and "why do some artists peak early, and others peak late?".
For example, in a paper titled 'Who were the great women artists of the 20th century?', Galenson tallied the number of illustrations of artworks by women in art history text books and found that "art historians judge Cindy Sherman to be the greatest woman artist of the twentieth century, followed in order by Georgia O'Keeffe, Louise Bourgeois, Eva Hesse, and Frida Kahlo."
You can read abstracts of a number of Galenson's papers on the National Bureau of Economic Research website.
David Galenson's papers