I think it's very difficult to come up with accurate objective measurement methods (nobody's done it well yet). The best assessments tend to be subjective teacher-based assessments (for similar reasons as to why hiring tends to be done by subjective personal assessment - it's hard if not impossible to define the criteria for "good" upfront. Someone may approach things in a completely different way that nobody had thought to measure). And this is before you get into the biases of the people drawing up the assessment criteria etc.