[R] simple main effect.

[Ista Zahn]

Hi Or,
I understand your question (and am not sure what the confusion is
about actually). Just like you said, you want to know the effect of A
at B=1, and the effect of A at B=2. In this case, you want to know if
drug has a significant effect for those with strain one, and whether
drug has a significant effect for those with strain two. So the good
part is, I understand the question.


The bad part is that unfortunately I don't know how to do it with
aov(). Also, I'm not a stats guru and could easily be wrong about the
following advice. You've been warned!

If you didn't have a mixed model, I would tell you to run the model
using lm() twice, setting drug = 1 as the reference group the first
time, and drug = 2 as the reference group the second time, but this
won't work in your case. The best I can offer is a suggestion to run
the model separately for each level of drug. Note that you can and
should use the error term from the overall model though -- you will
have to do this "by hand" (just divide  MS_effect from the subset
model by MS_error from the full model, and evaluate using df_error
from the overall model). So basically, I'm suggesting that you do

Data.drug1 <- subset(Data, drug == "1")
aov.model.drug.1 <- aov(dependent~(exposure*strain) +
Error(subject/exposure) + (strain), data=Data.drug1)
summary(aov.model.drug.1)

Data.drug2 <- subset(Data, drug == "2")
aov.model.drug.2 <- aov(dependent~(exposure*strain) +
Error(subject/exposure) + (strain), data=Data.drug2)
summary(aov.model.drug.2)

Good luck!

-Ista

On Sun, Dec 20, 2009 at 11:35 AM, Or Duek <orduek at gmail.com> wrote:
> For some reasion I wasn't able to use TukeyHSD - I think because I need to
> set the different levels under a second variable.
> Tukey only helps me when I have more than 2 levels of same variable.
> Thanl you.
> On Sun, Dec 20, 2009 at 6:32 PM, S Devriese <sdmaillist at gmail.com> wrote:
>
>> On 12/20/2009 04:56 PM, Or Duek wrote:
>> > I don't have missing data.
>> > about what I need.
>> > Lets say the drug*strain interaction is significant - now I want to check
>> > for drug under the levels of strain - compare drug 1 and 2 only on strain
>> 1
>> > and then only on strain 2.
>> > Or I'd like to compare the strains under levels of exposure.
>> > This is the kind of data I fail to see in summary() but it is important
>> to
>> > understand the interactions.
>> > thank you.
>> >
>>
>> Do you main pairwise multiple comparison tests like Tukey Honest
>> Significant Difference tests? Then you could use TukeyHSD in the stats
>> package or see the DTK package (Dunnett-Tukey-Kramer Pairwise Multiple
>> Comparison Test Adjusted for Unequal Variances and Unequal Sample Sizes)
>>
>> Stephan
>>
>
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>
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-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org