Horton's work raises many questions, not least because it contradicts other work suggesting that it is possible to improve poor workers' output by pairing them with good workers. By contrast, Horton found that "the bad apples ruined the good apples, and the good apples did nothing for the bad."
Here is much more of interest, on new developments in measuring worker productivity. In my view this effect is a significant factor behind the stickiness of wages. Negative signals often mean "get rid of the person" and not "renegotiate a lower wage." I thank an MR reader for the pointer.