From the intro: > The mathematical level of these notes is deliberately low; nothing should be beyond a competent third-year undergraduate. But every subject c

From the intro: > The mathematical level of these notes is deliberately low; nothing should be beyond a competent third-year undergraduate. But every subject covered here can be profitably studied using vastly more sophisticated techniques; that’s why this is advanced data analysis from an elementary point of view. "Elementary" is a relative term in my experience. The formula on 29 is the expected value of the quantity, (Y-m)^2. It is the average squared error under the distribution of Y incurred by a chosen value of m.