[R-sig-ME] [R] understanding I() in lmer formula
don-r-help at isis.cs3-inc.com
Wed Jun 14 18:23:11 CEST 2017
Dan Brooks writes:
> Related to your second question about ||, I believe || is shorthand for
> uncorrelating the slope and intercept, e.g:
> fm3 <- lmer(Reaction~Days+(Days+0|Subject)+(1|Subject),data=sleepstudy)
> fm4 <- lmer(Reaction~Days+(1+Days||Subject),data=sleepstudy)
> As opposed to:
> fm2 <- lmer(Reaction~Days+(Days+1|Subject),data=sleepstudy)
> or simply:
> fm1 <- lmer(Reaction~Days+(Days|Subject),data=sleepstudy)
Saying that it's shorthand for something else would be useful if I
understood what that something else meant!
I gather the formula language is intended to be intuitive, and maybe
it is for people who understand mixed models better than I do, but
to me fm3 and fm4 seem equally mysterious. The idea that days+1 should
be the same as days and that days+0 should be different from days seems
counter-intuitive to me.
And the distinction between correlated vs uncorrelated also still
has to be explained - I described some of my interpretation
problems in the previous post.
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