This is a decent way to estimate the average score. But sometimes you're not really trying to estimate the average score. You're really trying to estimate the score that the user who's currently viewing the page would give it, especially for someone like Amazon (ie, People Like You Rated this Product as 1-star).
His estimator might be a decent one at picking the average score, but in Amazon's case, it's not a great estimator of the rating I would give the product I am viewing. If you have such an extensive record of my past-purchases, use it to predict how I would like certain products! Surely the 5-star ratings of SICP are more applicable to me than the 1-star ratings.
His estimator might be a decent one at picking the average score, but in Amazon's case, it's not a great estimator of the rating I would give the product I am viewing. If you have such an extensive record of my past-purchases, use it to predict how I would like certain products! Surely the 5-star ratings of SICP are more applicable to me than the 1-star ratings.