However, the likelihood of selection effects in such studies has led many to question these results. (educationnext.org)
An attribution analysis confirmed that in fact most of the excess return came from selection effect, [3] in which active managers demonstrated their ability to pick winning stocks within each sector. (indexologyblog.com)
Given the data we use, we can't control selection effects that surely explain some of what we see here. (crpe.org)