Yes. It's typical for reviews like this to have certain criteria for the quality of a study that would or would not be included in a review. This would be the standard approach for any systematic review of the evidence; for example, it is what would be done at the International Agency for Research on Cancer, if they were reviewing a cancer endpoint. They have to look at the quality of the studies, and sometimes studies are included or excluded based on some kind of flaw in them.
On March 26th, 2013. See this statement in context.