Thanks a lot!
We figured out the dof :)
Hanne
Stephen Smith wrote:
> Hi - wrt questions 1 and 2, as far as I can tell this looks correct.
> Wrt question 3 - I'll leave that to you and your local stats guru to
> check out ;-)
> Cheers.
>
>
> On 11 Nov 2009, at 10:17, Hanne Lehn wrote:
>
>> Hi,
>>
>> Thank you very much for looking into this! I'm sorry to bother you
>> again, but
>> I'm not sure if I have done the analysis correctly. I would be very
>> helpful if you
>> or someone else could have a quick look and comment on what I did.
>>
>> Let me first remind you of the problem:
>>
>> I have 19 subjects, and each subject was scanned under 6 conditions. The
>> conditions correspond to three factors A, B, C. I want to analyze
>> these data
>> with a 3-factor (2x2x2) repeated measures ANOVA. I’m mainly
>> interested in
>> comparing the effect [B1C1>B2C1] across the two levels of factor A.
>>
>> PROBLEM: In two of my experimental conditions (cells), there are very
>> few
>> observations, and in some subjects none at all. To run the 3-factor
>> ANOVA, I
>> would have to exclude these subjects from the analysis. My question
>> was if
>> there was another way of testing the above interaction effect.
>>
>> WHAT I DID: I set up a 2x2 design instead. The design matrix has
>> inputs only
>> from conditions where C=1. This means I don’t have any empty conditions,
>> since these only occur when C=2 (conditions A2B1C2 and A2B2C2). And the
>> interaction effect AxB should equal “AxB given CV=1"), exactly what
>> I’m after.
>>
>> I followed the 2x2 ANOVA example on the website, just adding 19 EVs
>> with 1s
>> to indicate inputs from a given subjects. I use four inputs from each
>> subject.
>> These are the first level copes that correspond to the effect of each
>> condition
>> vs. baseline. I set up three F-tests, just like in the example, and
>> F3 is for the
>> interaction effect.
>>
>> QUESTIONS:
>> 1. Any caveats against testing a “reduced” design in the higher-level
>> analysis?
>> 2. Did I implement the analysis correctly?
>> 3. I get F with df1=1, df2=54 – would that be correct here?
>>
>>
>> Thank you very much in advance,
>>
>> Hanne
>>
>
>
> ---------------------------------------------------------------------------
> Stephen M. Smith, Professor of Biomedical Engineering
> Associate Director, Oxford University FMRIB Centre
>
> FMRIB, JR Hospital, Headington, Oxford OX3 9DU, UK
> +44 (0) 1865 222726 (fax 222717)
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