[R-sig-ME] meaning of lmer formula

Don Cohen don-r-help at isis.cs3-inc.com
Thu Sep 7 06:43:07 CEST 2017

Is there some documentation I should be reading about this rather 
than asking all these questions?  Other than the code, that is?

Ben Bolker writes:
 >  Separate terms in lme4 formulas are always independent.

In a formula like "out ~ in + ((a | group) + (b | group))"
do (a | group) and (b | group) qualify as separate terms?

 > (input1|group) + (input2|group) is problematic because both terms
 > include an intercept.

I was imagining that this duplication was removed after the formula
was expanded into some internal form (that I'd like to see).  I guess
you're saying that's not true.  

 > (0+input2|group) can be helpful, but doesn't do
 > what you think when the variable on the LHS (e.g. input2) is a factor.

Where can I read about what that means?

 > If input1 and input2 are both numeric (or 2-level factors) then they're
 > two independent 3x3s.

I have trouble seeing how factors make sense on the LHS.

Another question:
I tried an example with about a dozen inputs inside one group
(in1 + in2 + ... | group1) and another with the same inputs for
a second group, and both took about a minute, and then when I
used both groups the run time went up to about 10 min.
Is this expected and easily explained?

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