[R] Using MCMC sampling to estimate p values with a mixed model

[Robert A LaBudde]

If your alternative hypothesis is unequal variances (2-sided), both F 
< 1 and F > 1 are of interest, and rejection of the equal variance 
null can occur on either side.

The usual ANOVA F test is 1-sided, with an alternative the numerator 
variance exceeds the denominator one, so this is perhaps why you are confused.

At 02:40 PM 6/18/2011, Woodcock, Helena wrote:
>Hi everyone,
>
>Apologies if this is a silly question but I am a student and this is 
>my first time using R so I am still trying to educate myself on 
>commands, models e.t.c
>
>I have a mixed model with four dichotomous fixed factors and subject 
>as a random factor (as each person completed four vignettes, with 
>factors crossed across vignettes).
>
>I have run an lmer model and used the Monte Carlo method to see if 
>there are any significant main effects or interactions. However, 
>when I looked at the p values some are showing as significant 
>although the F value is less than 1. Is it possible to have a 
>significant effect with an F value below 1?.
>
>I have a sample size of 150 and have read that the pMCMC values can 
>be anti-conservative so wonder if it is because my sample size may 
>be too small?.
>
>Thank you for any help
>
>         [[alternative HTML version deleted]]
>
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================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
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