Dear Geoff,
> > For SPM{T} with large d.f. the approximations for SPM{Z} are sufficient
> > and are therefore used
>
> Are you advocating some concrete threshold of df? and is this
> hard-coded into SPM? I ask only because of the increasing prevalence of
> random effects analyses that have much reduced degrees of freedom.
There is no hard coding in SPM99. Your point about second-level
analyses is a good one (I had overlooked this and was assuming that all
analyses would now have large d.f., ideally more than 32 but certainly
more than 16).
Spatial extent tests are generally more powerful when the resolution of
the data is small in relation to the size of the activations (this is
generally the case for fixed-effect, subject-separable analyses with,
say, 4mm smoothing).
Height-based tests are more powerful for low resolution data (e.g. PET
or random-effect analyses that usually employ higher degrees of
smoothing to accommodate inter-subject variability in functional
anatomy, say 6-8mm). I think therefore that most people will use
corrected inferences based on height for second-level analyses and this
should certainly be the case when the d.f. are small (less than 16).
Any thoughts?
With very best wishes - Karl
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