[R-sig-ME] lme4: constrain sigma to 0 [SEC=Unclassified]

[Steve Candy]

One approach is to be able to constrain sigma to 1 where this fits into a strategy of estimatiing all higher-level variance components as "gammas" which are the ratios of these variance components to sigma^2. Sigma can also be estimated or fixed to a constant.

This makes sense when lmer or lme is used within a PQL algorithm for fitting a GLMM with binomial response.

I am not sure setting sigma to zero is useful since there will always be some level of units-level variance but it would be good to have the option of constraining it to some positive nonzero value.
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