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str – General variance structures


str is a general variance structure spanning consecutive model terms.


str(form, vmodel)

form A model formula included verbatim in the asreml() random argument.
vmodel A direct product variance model for the set of terms given in form.

Typically, a variance structure applies to an individual term (main effect or interaction) in the linear model, and there is no covariance between model terms. Sometimes it is appropriate to include a covariance, such as random coefficients regression, for example. In such cases it is essential that the model terms be contiguous and that the variance structure defined is the structure required across all terms in the set. The model terms in form are consequently not reordered. While asreml will check the overall size of the included terms, it cannot check that the order of effects matches the structure definition in vmodel; care must be taken to ensure this is correct. Check that the terms are conformable by considering the order of the fitted effects and ensuring the first term of the direct product in vmodel corresponds to the outer factor in the nesting of the effects in form.

Updated on June 22, 2021

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