There are, in general, an uncountable number of quadratic forms that can be used for variance components estimation. Any set generating a H with full rank will provide unbiased estimates and there are no clear-cut rules for choosing among sets (Searle 1971: 455). Normal practice is to use the 'sum-squares' obtained from an analysis-of-variance model of each effect con sidered alone. In this model the presence of the C,,,13 term required that an additional quadratic form be defined