 |
 |
Maybe of interest >Dear Mly, > >I think you are absolutely right. We DO assume that there is a common component >in the contrasts. Also I think that we do not need to assume anything about the >effect size in the different contrasts, and in that case it would be kosher. Why >don't you send your mail to the mailbase, I think it would be very useful. > >Puss Jesper > >Emiliano Macaluso wrote: > >> Dear Jesper, >> >> happy new year. >> >> As usual I find your e.mails the most "enlightening"! >> >> My impression on the matter is that it is true >> that we usually use 5 different treatments A-E, >> but all of these have something in common. >> Say A-E are different tasks, all of which share >> a common cognitive component "X". >> >> For a brain area that respond to X, >> we are effectively testing 5 time the same effect (X). >> >> In this case it seeems to me >> that the min-t-stats would be approprate! >> >> What do you think? >> >> cheers, >> >> mly >> >> At 12:24 PM 1/11/2001 +0100, you wrote: >> >Dear Pierre, Richard, Joe and everyone, >> > >> >while certainly not being an expert, I thought I should add my five cents to >> >this very interesting discussion. >> > >> >Joe gave a nice explanation of deMorgans inequality, which states that if you >> >have a set of statements then the "set" is true only if all the statements >> >are true. Or conversly the "set" is false if one or more of the statements >> >are false. >> > >> >I think this is in fact precisely what Pierre and Richard (and I) have a >> >problem with. That means that we (might) reject the null hypothesis if a >> >single one of the individual hypotheses is wrong. This is certainly not in >> >accordance with my intuitive interpretation of "conjunctions" which I have >> >thought of as testing if they are all wrong (i.e. if we have "activations" in >> >all of the constituent contrasts). >> > >> >When does one use a "minimum" statistic? Well, say that we have 5 independent >> >samples of THE SAME effect, for example we have tested treatment A in 5 >> >groups of 10 subjects. The null hypothesis is that A doesn't work, and we >> >might proceed to test this with the minimum t-statistic of these 5 different >> >t values. >> > >> >Now, say instead we have 5 independent samples testing DIFFERENT effects, for >> >example treatments with compounds A to E in groups of five subjects. The null >> >hypothesis is that none of the compounds work, and again we use the minimum >> >t-statistic to test it. I suggest that if we reject the null hypothesis we >> >conclude that at least one of the compunds work. But I suspect we cannot say >> >that all compounds work, which is what we would say in "neuroimaging >> >conjunctions". >> > >> >So, isn't perhaps the problem that an assumption (one which we never test) in >> >the use of the minimum t-statistic is that we test THE SAME effect in all >> >tests? Whereas this in fact ought to be the hypothesis that we test. >> > >> >I hope I haven't added to the confusion too much with this. >> > >> >Jesper >> > >> > > > >