Nicole Krämer, Masashi Sugiyama, The Degrees of Freedom of Partial Least Squares Regression, Journal of the American Statistical Association, Vol. 106, No. 494 (June 2011), pp. 697-705 ...

In calculating the variance of a mean or in constructing an approximate F-test, it is frequently necessary, particularly with unbalanced data, to form a linear function of mean squares, MS=Σ α iMSi, ...