Re: [R] interpreting mixed model anova results

From: hpdutra <hpdutra_at_yahoo.com>
Date: Sat, 05 Jul 2008 23:34:32 -0700 (PDT)

Sounds like a repeated measures design, if so then you have to incorporate this into your model before interpreting the results.

Brooke LaFlamme-2 wrote:
>
> Hi, I am trying to do a repeated measures ANOVA to determine if there is a
> significant difference between two sets of timecourse data. Each
> individual
> was given a single treatment and then measured for one variable for 10
> days.
> Here is made-up example of what my data would look like:
>
> data<-data.frame(subject=rep(c("A1","A2","A3","B1","B2","B3"),10),treatment=rep(c("A","B"),each=3),
> day=rep(c(1:10),each=6),response=rnorm(60))
>
> This is the code I run to test for a difference between treatments A and B
> over the course of the 10 days:
>
> aov(response~day*treatment+Error(subject), data=data)
>
> I believe this is the correct model to use, though I could definitely be
> wrong.
>
> Here is the output I get from my actual data (using summary(aov)):
>
> Error: subject
> Df Sum Sq Mean Sq F value Pr(>F)
> treatment 1 4258.1 4258.1 12.588 0.001344 **
> Residuals 29 9810.2 338.3
> ---
> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
> Error: Within
> Df Sum Sq Mean Sq F value Pr(>F)
> day 9 98345 10927 150.313 < 2.2e-16 ***
> day:treatment 9 6844 760 10.461 8.374e-14 ***
> Residuals 261 18974 73
> ---
> Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
>
>
> The p-value for "treatment" is the same as what I would get if I lumped
> the
> data from all 10 days together, so I assume this is not what I want here.
> However, am I correct in interpreting the p-value for day:treatment as
> what
> I want? Does this tell me that there is a difference between the two
> groups
> over the course of the 10 days (regardless of which days actually are
> different) with respect to the fact that I am measuring the same subjects
> each day?
>
> Thanks for any help!
> --
> Brooke LaFlamme
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
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>
>

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Received on Sun 06 Jul 2008 - 06:39:07 GMT

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