The Friston et al. (1999) NeuroImage paper on using multiple conjunction
analyses in a fixed-effects analysis suggests that such analyses, "might
be a useful prelude to studies of large numbers of subjects that are
required by random-effects analyses". However, couldn't one also use a
regular fixed-effects analysis (not involving conjunctions) in the same
manner, as a pilot study preceding a larger random-effects analysis?
For example, one could do a pilot study with a small number of subjects,
run a fixed-effects analysis, identify regions of activation, and then use
these activation areas as regions of interest for a subsequent random
effects analysis with a different group of subjects. The advantage would
be that one could use a less stringent criterion for the random-effects
analysis for the regions of interest (say, p < .005 vs. p < .001).
Is this a valid procedure? I have seen one fixed-effects analysis used to
produce hypotheses for a subsequent fixed-effects analysis (for example,
Henson et al. 2000, Science, expts. 1 and 2 were thresholded at .001 but
because of the hypotheses generated by these two expts. the two subsequent
experiments were thresholded at .005), which was then followed by a
random-effects analysis over all the 4 component fixed-effects studies.
However, the fixed-effects results were (as far as I can tell) not used to
generate hypotheses to relax the significance criterion for the
random-effects analysis.
One drawback I could see with using a fixed-effects pilot study followed
up by a random-effects study is that the fixed-effects subjects could have
been allocated to the random-effects study in the first place, which would
increase the power of the random-effects analysis. It's unclear whether it
is better to just increase the N for the random-effects analysis or do a
two-stage approach with a fixed-effects hypothesis-generating stage
followed by a random-effects analysis with an independent group -- any
ideas?
As always, any help very much appreciated.
Stephan Hamann
Dept. of Psychology
Emory University
Atlanta, GA 30322
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