Hey Andreas & Manfred,
Couldn't you simply use a one-way ANOVA, which is equivalent to a
two-tailed t-test when there are only two conditions? Then you'd obtain
two-tailed p-values and you could apply your desired significance
thresholds directly.
Good luck & best,
Michael
On 08/01/18 14:47, Manfred Klöbl wrote:
> Dear Andreas,
>
> as far as I know, the threshold of 0.05 is completely fine here, also
> for two-tailed situations. This leaves 0.025 at each tail of the
> distribution (someone with more insight into the exact SPM statistics
> implementation might correct me if I'm wrong). You get the complete
> statistics for both contrasts, just the signs are inverted and SPM does
> not display negative significant results even if they are there. You can
> check that easily using image viewers like MRIcro with your t-maps.
>
> Best regards,
> Manfred
>
>
> Am 08.01.2018 um 14:30 schrieb Andreas Jansen:
>> Dear colleagues,
>>
>> I am interested in activation differences between two conditions
>> (contrasts active > baseline, baseline > active). For that, I used an
>> one-sample t-test and a threshold of p=0.001 at the voxel level,
>> p<0.05 at the cluster level. Now someone has correctly pointed out
>> that I use a typical one-tailed threshold for a two-tailed test. What
>> threshold do I have to use? Do I just have to correct the cluster
>> level threshold, i.e., p=0.001 at the voxel level, p<0.025 at the
>> cluster level for each of the two contrasts?
>>
>> Best wishes Andreas
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