[R-sig-ME] meaning of lmer formula
Ben Bolker
bbolker at gmail.com
Wed Sep 6 22:45:32 CEST 2017
On 17-09-06 03:57 PM, Don Cohen wrote:
>
> My current understanding is that something like (input2 | group)
> means the same as (1 + input2 | group) which means to estimate
> for each group an intercept, a slope and a correlation between the two.
Yes.
>
> Does (input1 + input2 | group) mean the same as
> (input1 | group) + (input2 | group) or does it also include a
> correlation between input1 and input2 ?
The latter.
>
> Back to my question about additive shifts, is that extra correlation
> required in order to maintain invariance over additive shifts?
Yes.
>
> Is there some way in the formula language to control more precisely
> which correlations are to be included/excluded?
>
Unfortunately not.
I gave an example here
https://stackoverflow.com/questions/38976189/syntax-of-pdblocked-to-specify-covariance-matrix-in-mixed-effects-model-nlme
about how to specify some structural zeros in the covariance matrix, but
it's not easy ...
http://rpubs.com/bbolker/varcov_equiv may be useful as well.
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