In our June 2002 sample period, 28 unique mySimon
retailers carried at least 1 of the 1,640 books. All books in
the sample were offered by multiple retailers. To compare
retailers that carry a similar assortment of books, we ranked
the booksellers with respect to the percentage of books they
carried from the total list of 1640. We observed a gap between the top 14 retailers, which carried at least 40% of
the books, and the bottom 14 retailers, which carried, at
most, only 9% of the books. On the basis of this observation,
we limit our analyses to the 14 retailers that carried at
least 40% of the books. The final sample of retailer prices
has 392,245 total observations, or an average of approximately
28,000 per retailer