Daniel Willingham is a very smart and sensible psychologist at the University of Virginia. He has a talent for explaining complex issues in simple language.
In this video, he gives six reasons why value added assessment and merit pay are unfair–all in three minutes.
Diane Ravitch's blog 
A persuasive piece from the brilliant Dan Willingham. Gotta love the format, too, which is so clear and engaging that even a politician might be able to get it.
What’s ironic is that people talk about getting more money into the classroom. All this data analysis that is now in vogue isn’t done in a classroom, and it’s not going to be done for free.
Maybe even Arnie could understand it.
I can think of even more factors that make VAM unfair…students who receive lots of support at home versus those who don’t and kids who are in school every day versus those with terrible attendance, to name two. As far as I know, these factors are not included in the VAM formula, although than can make a HUGE difference in how a child does on a test, AND THEY ARE NOT UNDER A TEACHER’S CONTROL. VAM, as it currently exists, is junk science with such a low reliability rate it is shameful. In the next few years, I guarantee we see many otherwise highly regarded teachers labelled ineffective, and possibly losing their jobs, because of it. Just as scary is the prospect that poor or mediocre teachers will receive raises and accolades just because their students happen to have high VAM scores through no merit of their own. I’m scared for the future of my profession.