[R] lmer for two models followed by anova to compare the two models
Lawrence Hanser
larry at hanser.net
Thu Oct 16 16:29:23 CEST 2008
Dear Colleagues,
I run this model:
mod1 <- lmer(x~category+subcomp+category*subcomp+(1|id),data=impchiefsrm)
obtain this summary result:
Linear mixed-effects model fit by REML
Formula: x ~ category + subcomp + category * subcomp + (1 | id)
Data: impchiefsrm
AIC BIC logLik MLdeviance REMLdeviance
4102 4670 -1954 3665 3908
Random effects:
Groups Name Variance Std.Dev.
id (Intercept) 0.11562 0.34003
Residual 0.22765 0.47713
number of obs: 2568, groups: id, 107
run this model (only difference is I've removed the interaction term):
mod2 <- lmer(x~category+subcomp+(1|id),data=impchiefsrm)
obtain this summary result:
Linear mixed-effects model fit by REML
Formula: x ~ category + subcomp + (1 | id)
Data: impchiefsrm
AIC BIC logLik MLdeviance REMLdeviance
3987 4151 -1966 3823 3931
Random effects:
Groups Name Variance Std.Dev.
id (Intercept) 0.11528 0.33953
Residual 0.23584 0.48564
number of obs: 2568, groups: id, 107
Note that the loglik from the first model is -1954 and from the second
model loglik is -1966.
Next, to test the difference between the two models I run:
anova(mod1, mod2) and obtain this result:
Data: impchiefsrm
Models:
mod2: x ~ category + subcomp + (1 | id)
mod1: x ~ category + subcomp + category * subcomp + (1 | id)
Df AIC BIC logLik Chisq Chi Df Pr(>Chisq)
mod2 28 3879.1 4042.9 -1911.5
mod1 97 3859.3 4426.9 -1832.7 157.72 69 6.71e-09 ***
---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Note that in this result the logLiks are reported as -1832.7 and
-1911.5 respectively for models 1 and 2.
Now my question:
Why are the logLiks from the anova command that compares the two
models different from what was reported in the separate model results?
Thanks,
Larry
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