[R-sig-ME] Modelling random effects for only part of the observations (in lme4)

[Hufthammer, Karl Ove]

Thierry ONKELINX wrote:
> Dear Karl Ove,
>
> (X|G) is equivalent to (1 + X|G). Or mathematically: b_0i + b_1iX. But you need b_1iX.
> 
> The solution is to remove the random intercept (0 + X|G)
> 
> I would go for lmer(y ~ arm + (0 + b|gr2))

Great! Thanks. This works perfectly.

And I now understand what's going on in the formula too. The lme4 syntax is rather elegant. :)

The fixed effect estimates are near identical to estimates based on lm(y ~ arm), while the standard errors are different, which is exactly what would be expected. The reason they're not exactly identical is that one treatment group had only 2 subjects, so the design was not perfectly balanced. If I change it to a balanced design (equal number of subjects in each treatment group), the estimates are identical between lmer and lm (while of course the standard errors still differ).

-- 
Karl Ove Hufthammer