Failed to get ‘positive outliers’- what is "positive"? what is "outlier"?

Context:

Despite their seeming ubiquity, quantifying bad apples is an understudied area.

“Most of the work in organization design and human resource management has been focused on what I would say are ‘positive outliers’ — the really top performers,” or star talent, said economist Dylan Minor, a visiting assistant professor of business administration at HBS and the paper’s co-author. “[As] it turns out, we’ve all had personal experiences where we have a worker on the other side of the distribution [who], rather than really helping performance, actually hurt performance in one way or another.”

More:
http://news.harvard.edu/gazette/story/2015/11/those-toxic-co-workers/