On the other hand, many of us also like to try to judge a trade as it happens. Everyone does this, some using a qualitative approach and others using a quantitative approach (e.g. my analysis of the Rolen trade). Yet some would say that we shouldn't evaluate a trade until after the fact, as there's just too much uncertainty in trying to forecast the future--and what matters are results, right?
I certainly agree that there are big error bars around quantitative trade analyses. But I still think there's a lot of merit in trying to judge trades as they happen, because it helps us recognize quality moves by GM's--regardless of the eventual outcome. You can make what, by all information available, is a great trade at the time, only to have some bad luck (injuries, etc) derail it in a post-hoc analysis. I still think we should recognize that the GM made a smart move, even if it falls apart later.
Paul DePodesta wrote a great article about this that I've been meaning to link to for a long time. Here's an excerpt:
As tough as a good process/bad outcome combination is, nothing compares to the bottom left: bad process/good outcome. This is the wolf in sheep's clothing that allows for one-time success but almost always cripples any chance of sustained success - the player hitting on 17 and getting a four. Here's the rub: it's incredibly difficult to look in the mirror after a victory, any victory, and admit that you were lucky. If you fail to make that admission, however, the bad process will continue and the good outcome that occurred once will elude you in the future. Quite frankly, this is one of the things that makes Billy Beane as good as he is. He is quick to notice good luck embedded in a good outcome, and he refuses to pat himself on the back for it.