The exact method for compiling the data obtained from the booksellers is classified as a trade secret. Book Review staff editor Gregory Cowles explained the method "is a secret both to protect our product and to make sure people can't try to rig the system. Even in the Book Review itself, we don't know (the news surveys department's) precise methods." In 1992, the survey encompassed over 3,000 bookstores as well as "representative wholesalers with more than 28,000 other retail outlets, including variety stores and supermarkets." By 2004, the number was 4,000 bookstores as well as an unstated number of wholesalers. Data is adjusted to give more weight to independent book stores, which are underrepresented in the sample.