Stone 1974 is referenced in:
Michaelsen J. 1987. Cross-validation in statistical climate forecast models. J Climate Applied Meteorology, 26:1589-1600


Set Z = z_{1}, z_{2},... , z_{1} consists of predictions and targets z_{i}=(x_{i}, y_{i})

A set of prediction rule \eta (x,Z) will be used to predict y0 from \eta (x_{0},Z)

Let Q(y_{i},\eta_{i}) be the accuracy.
by least squares this will usually (y_{i}-\eta_{i})^{2}
in other words expected Err is
Err= E[Q(y_{i},\eta(x_{0},Z))]


MSE= \sum_{i=1}^{n}Q(y_{i},\eta(x_{i},Z))/n

In cross validation
MSE_{(CV)}= \sum_{i=1}^{n}Q(y_{i},\eta(x_{i},Z_{(i)}))/n