Adding resources to a late project makes it later. -- Frederick P. Brooks, Jr.
from "The MythicalManMonth," (20th Anniversary Edition), 1995, Addison-Wesley.
There is a model (probably several) for Brooks' Law. I used one of them for creating the pattern ProgressAndTrainingTeams in the RiskManagementCatalog. The model more or less says (as Brooks did) that staff quality and productivity per person both go down as people go up. The part he left out is that part of the productivity loss is from training. The pattern tries to fix that by putting training elsewhere than where the experts are busy trying to get work done.
-- AlistairCockburnThe part he left out is that part of the productivity loss is from training. - I think you'll find that's not the case. My copy is currently out on loan, but I'm pretty sure he explicitly says that part of the reason you fall further behind is because now there's more work to do - namely - training.