Hello SPM fans,

I calculated a two sample t-test using data from young and old subjects and I got a strange result.

If I look at the data for either the young or the old population in the two sample t-test scenario, I get almost no difference.
If I calculate a one sample t-test for each group, I get a significant difference in activation with different T thresholds. Furthermore, when I compute the difference between the two in the two sample t-test, I get no significant activation, which should make sense, because there is no difference between the two contrasts inside the two sample t-test calculation. Additionally, the cutoff threshold for both is the same using the two sample t-test, which I find strange.

I've attached some screen captures so you can see what I mean.

The design matrix shows that for group 2 (older subjects) the box is gray. What is happening here?

I did two measurements exactly two years apart using the same design. Is there a problem of independence?

I also preprocessed them the same, using new segment and dartel normalization.

Cheers,
Glad
<old_one_sample_ttest.png><old_two_sampled_ttest.png><young_one_sampled_ttest.png><young_two_sampled_ttest.png>