Hi Torben,
thanks for your quick reply. So the two data sets do indeed have unequal
variances, which is good, because they don't violate the assumption of
sphericity. However, I don't understand why both contrasts:
[1 0] for young subjects
[0 1] for old subjects
result in identical contrasts with identical thresholds, while looking
at them from an one sampled t-test perspectives gives different results.
Any thoughts?
Regards,
Glad
On 09.01.2014 12:59, Torben Lund wrote:
> Dear Glad Mihai
>
>
> The gray box indicates that you have specified that the two groups have
> unequal variance. These variances is estimated by SPM and if they are
> indeed different you will see different gray-levels, because the
> designmatrix have now been whitened. You can also inspect the covariance
> matrix directly, check the SPM manual for instructions on how to do this.
>
>
> Best
> Torben
>
>
> Torben Ellegaard Lund
> Associate Professor, PhD
> Center of Functionally Integrative Neuroscience (CFIN)
> Aarhus University
> Aarhus University Hospital
> Building 10G, 5th floor, room 31
> Noerrebrogade 44
> 8000 Aarhus C
> Denmark
> Phone: +45 7846 4380
> Fax: +45 7846 4400
> http://www.cfin.au.dk <http://www.cfin.au.dk/>
> [log in to unmask] <mailto:[log in to unmask]>
>
>
>
>
>
> Den Uge:2 09/01/2014 kl. 12.40 skrev Glad Mihai
> <[log in to unmask] <mailto:[log in to unmask]>>:
>
>> 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>
>
--
Glad MIHAI, M.Sc. Biomedical Physics
Functional Imaging | University Clinic Greifswald
Walther-Rathenau-Straße 46 | 17475 Greifswald | Germany
Tel: +49 3834 86 69 44 | Fax: +49 3834 86 68 98
www.baltic-imaging-center.de
|