Dear Julia,
I assume you are referring to the Misc tab in the FEAT GUI
and in particular to the "Z Threshold" within the "Design efficiency"
part. This is not a threshold that is used to calculate your
activation statistics in any way, so do not be concerned. This
simply refers to the efficiency calculation (like a power calculation)
that is presented with the design matrix, showing the % BOLD signal
change required for each contrast. I would leave this at its default
setting as it gives you a reasonable estimate for efficiency (relating
more to the sensitivity to real effects rather than the null-hypothesis
false positive rate).
The actual thresholds for your statistics are found in the Post-stats
tab and are controlled via the corrected p-values (also with an
arbitrary z-threshold if cluster-based correction is used). It is only
the threshold in the Post-stats tab which will be used on your data
to determine statistically significant activation.
All the best,
Mark
On 29 Jun 2011, at 13:32, Julia Foecker wrote:
> Hi,
>
> I have a question concerning the Misc option in FSL.
> The z-threshold was preset to 5 (or 5.8).
>
> How do I know which is the best threshold for my data? (I think the 5.8 is much
> too high...).
> And are there any values which should not be used because they are too low?
>
> Thanks a lot,
> Julia
>
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