In Think Like a Freak, Levitt and Dubner write, "The key to learning is feedback. It is nearly impossible to learn anything without it" (34). A few paragraphs later they add, "But the more complex a problem is, the harder it is to capture good feedback." I'd add this goes for all parties in the process.
Therein lies the entire problem with assessment/standardization/data/value-added issue in education in a nutshell. It's just too wonderfully complex.