Your reviewer wonders whether SPM2 can compare 2 vs 16. Clearly it can
do so, since you have done it. The question is whether it is optimal to
do so. There are alternative models, for example:
1. one 2 vs 16 2-sample t-test as you have done.
2. two separate 1 vs 16 2-sample t-test, treating the patients as a case
and replication, with each patient group having n=1. (note that this is
not the same as a one-sample t-test of the differences between your
patient and each control: this should be avoided)
3. a 1x3 anova, with groups 'controls n=16' vs 'patient n=1' vs 'other
patient n=1', enabling you to look at individual or averaged
differences between patients and controls.
The choice depends on the inferences you wish to make, the flexibility,
and the assumptions you are prepared to make. For your model, you
should be able to state that the first level models for patients were
similar to controls, in terms of number of scans, covariates and
residual variance, since large differences in the first level (e.g. due
to many more patient errors or worse patient movement) would undermine
the assumptions behind the two-step random effects approach used in
SPM2 (since it is not a true mixed effects model).
With your 2 vs 16 model in SPM 2 you are assuming that the {expected}
error variance of you patients is not different from the control group.
This might be contributing to your reviewer's unease. If you want to
avoid many of these assumptions, you could have a look at SnPM3. Or to
allow for unequal variances you could look at SPM5.
Best wishes,
James
>>
>> we have conducted a study comparing 2 rare patients against 16
>> controls. We have the same contrast (lets call it C) in all 18
>> subjects at the first level. At the second level I performed a
>> 2-sample t-test (voxelwise) with 2 subjects in group 1 and 16
>> subjects in group 2. One of the reviewers states:
>>
>> Regarding the 2 sample t-test. I wonder if SPM2 is able to compare 2
>> subjects versus 16. I believe this is not the case and that one must
>> use a different methodology to make this comparison. I would like to
>> see a citation to a study that uses a similar approach that might
>> alleviate this concern.
>>
>>
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