Why are some listings included in the Supply Demand Report and not others?

The Supply Demand Report is a cohort analysis of top ranked fixed priced, multi-quantity listings that have been out on the marketplace for the last 18 months.  They have gained enough exposure to be in the top rank and have collected sales to prove buyer demand. Studying this group of listings give us a good estimation of potential sales velocity.

Excluded are auctions, single quantity listings and listings older than 18 months for the fact that they skew the data.