Explain This Regarding Publisher's Weekly Yearly Lists

I was looking through the Wikipedia at the top selling novelsper year as defined by Publisher’s Weekly

But the article states:

What does this mean exactly? I can’t find out where it explains what the standards to be in the list are in the first place.

Any ideas?


As per Publisher’s Weekly themselves:

Other lists have dealt with the success of Harry Potter in an interesting way: due to the Harry Potter books remaining at the top of the charts, the New York Times had to create a separate list of bestselling childrens’ novels; and then had to separate that one into two lists- one for one-offs and one for continuing series- because Harry Potter kept hogging the top of the childrens’ list as well.