Thanks Andreas, I was trying to find this link with no success!On 3 December 2016 at 10:27, Andreas Bartsch <[log in to unmask]> wrote:Hi,also note that F-tests have a spherical or elliptical rejection region while individual t-tests evaluate rectangular rejection regions. This leads to differences in the rejection regions: points outside an ellipse may be detected by the F-test but, if they fall within a box, the H0 may not be rejected by the t-test. See http://www.amstat.org/publications/jse/v16n3/martin.html (Tom Nichols alerted me once to Fig. 2). It is a bit of a different plot than your example (because you are wondering about the discrepancy of positive detections) but it may be worthwhile to mention.Cheers,AndreasVon: FSL - FMRIB's Software Library <[log in to unmask]> on behalf of "Anderson M. Winkler" <[log in to unmask]>
Antworten an: FSL - FMRIB's Software Library <[log in to unmask]>
Datum: Samstag, 3. Dezember 2016 um 10:49
An: <[log in to unmask]>
Betreff: Re: [FSL] Discrepancy between f- and t-test imagesHi Lisa,Oops, my bad, it's Hayter, not Hayer: Hayter AAJ. The maximum familywise error rate of Fisher’s least significant difference test. J Am Stat Assoc. 1986 Dec;81(396):1000–4. http://www.jstor.org/stable/2289074 However you don't have to bother much with it -- it refers to differences more in the opposite direction, i.e., excess false positives as opposed to not finding results in the t-test, and also it shows that with 3 groups it's fine. The fact that you are using cluster-level inference already explains the issue: there is no guarantee that the F-test will match the t-tests even in the 3-group case.Which to choose then? I would go for the one that has higher specificity: the t-tests. I would leave the F-test results aside.All the best,AndersonOn 2 December 2016 at 12:28, Lisa Kramarenko <[log in to unmask]> wrote:And I have another question: would you mind giving a hint about how to interpret the not-matching results? Is it a difference to be reported or rather not?Unfortunately I can't find the paper you're referring to, could you also tell me its name? Or maybe you can really briefly summarize what the reason for this discrepancy is?Hi Anderson,thanks for your response! Indeed, I am comparing three groups and I am using easythresh (after flameo) which performs multiple comparison correction at the cluster level!Thanks so much for your help!On 1 December 2016 at 10:16, Anderson M. Winkler <[log in to unmask]> wrote:Hi Lisa,The results may not match, particularly if:- Spatial statistics are used (e.g., cluster, TFCE).- The rank of the F-test diverges from 2 (e.g., if you are comparing 3 or more groups). For a reference, see Hayer (JASA, 1986).All the best,AndersonOn 30 November 2016 at 10:26, Lisa Kr <[log in to unmask]> wrote:Hello,
I have some trouble understanding the discrepancy in the results of my ANOVA. While the F-stat image shows the regions with significant differences between my three groups, the individual t-tests then show that the differences between the single groups are in regions other than these F-test has shown (see attached, red=f-test, green and blue: group differences). Is there any explanation for this? I was expecting the differences between groups to fall in the general areas indicated by the F-Test.
Thanks!!
Lisa