but we use Student Growth Percentiles!

and they aren't any better at measuring "teacher input."

Unfortunately, while SGP*s are becoming quite popular across states including Massachusetts, Colorado and New Jersey, and SGPs are quickly becoming the basis for teacher effectiveness ratings, there doesn’t appear to be a whole lot of specific research addressing these potential shortcomings of SGPs. Actually, there’s little or none! This dearth of information may occur because researchers exploring these issues assume it to be a no brainer that if VAMs suffer classification problems due to random error, then so too would SGPs based on the same data. If VAMs suffer from omitted variables bias then SGP would be even more problematic, since it includes no other variables.

Unfortunately, indeed, as we're doing it, anyway.

*SGP: student growth percentiles
VAM: value-added measure