As I was sorting through my fan mail yesterday, I came across a question from alert reader Joe Statin, asking whether it is OK to combine data from subjects with different scanning parameters; for example, if ten subjects were scanned with a TR of two seconds, and the other ten subjects scanned with a TR of four seconds, with everything else being equal. There are no definite answers for this, but in general, you need to ask yourself whether the difference in scanning parameters would make much difference. In the case of different TRs, a longer TR would result in slightly higher signal-to-noise ratio (SNR), but other than that, would not affect much else. (It would be much more difficult to carry out a meaningful finite impulse responses analysis looking at each acquisition individually, but in this case let's say we are dealing with simple contrasts across regressors.)
One way you can test this quantitatively is by directly comparing the beta estimates from one group of subjects, and comparing it to the other group of subjects that was scanned differently. If it doesn't appear to make much of a difference one way or the other, it is probably OK to combine both groups; if, however, there is a large difference, or if the results appear to be highly sensitive to whether someone was scanned with a two-second TR or a four-second TR, then you may want to hold off on combining them.
Combine the groups, and you will regret it; do not combine the groups, and you will regret it. Combine the groups or do not combine the groups, you will regret it either way. This, gentlemen, is the sum and substance of all philosophy.
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