I think that would be relatively easy to estimate from Amazon's rating since they break down the votes. It's a safe assumption that 1 or 2 star votes would be e

I think that would be relatively easy to estimate from Amazon's rating since they break down the votes. It's a safe assumption that 1 or 2 star votes would be equivalent to a down vote, and 4 or 5 star votes would garner an up vote.