Determining a bestseller book list is a tricky business, and depends largely on the organization creating the list and what factors they choose to measure. When people talk about "the" bestseller list, they are usually referring to New York Times (NYT) bestselling books, but there are many caveats. For instance, the NYT list only takes into account how books are selling in national and independent bookstores that specialize in selling books; it ignores all sales from department stores like Walmart in the calculations. The Wall Street Journal, USA Today, Publishers Weekly and other famous publications also have their own bestseller lists, which are all formed in different ways and often have very different books on them.
Compiling the Lists
Many bestseller lists are created or compiled by separate parties from those who publish them -- the NYT list, for instance, is created by the News Surveys department but published in the Book Review department. Sales are tracked week by week, which means that books which sell quickly often rate higher than books that sell more slowly over a period of time. A book could have steady sales for years, but not rise very high on the NYT list because sales week by week are relatively low. In this way, bestseller lists are often seen as short-term tools that show what books are immediately popular.
Bestseller lists are also often divided between certain types of books to help clarify. The New York Times Bestsellers are divided into fiction and nonfiction, with a children's section added in recent years, but this is only the beginning. Many lists only focus on a specific genre or one bookstore business.
Books on the List
While on the surface the lists are very basic, going simply by number of sales within a given time period, many underlying factors -- like advertising and store placement -- also play into the mix. When it comes to pure sales in the short term, books that are well advertised and noticeable (i.e., placed in store windows or up front in separate book stands) have a much higher chance of landing on a bestseller list. Prominent book reviews (both good and bad) and the number of retailers involved in selling the book also determine its success.
As people become more aware of these external factors, it has become popular for bestseller lists to add extra information, such as how many weeks the book has been in the top 10 or top 100, what place the book has had in past weeks, when the book entered the bestseller list and what its peak position was.