[FieldTrip] second-level statistical inference
Stolk, A.
a.stolk at fcdonders.ru.nl
Fri Nov 25 14:32:39 CET 2011
Hey Sara,
Yes, it is ok to do it like this (replacing every stat field by a 'avg' field). We'll fix this asap possible. Id est, to make ft_timelockgrandaverage to do this for you. Sorry for the inconvenience.
With repect to your last step (step 4), a depsamplesT t-test is suitable for testing varibles that depend on the same factor (within-subjects effects). The answer is 'yes'. :)
Best regards,
Arjen
----- "Sara Bögels" <s.bogels at psy.gla.ac.uk> schreef:
> Van: "Sara Bögels" <s.bogels at psy.gla.ac.uk>
> Aan: fieldtrip at donders.ru.nl
> Verzonden: Vrijdag 25 november 2011 12:51:32
> Onderwerp: Re: [FieldTrip] second-level statistical inference
>
> Hi Arjen,
>
> Thank you very much for your answer. That sounds good, but step 2 does
> not work straightforwardly, since matlab gives the error message that
> it
> cannot find an avg field (which would not be in the structure created
> by
> ft_timelockstatistics). Just saying cfg.parameter = 'stat' does not
> work. I tried to get around that by inserting an avg field which is
> the
> same as the stat field for each participant. Matlab also asked for an
> fsample field, which I inserted from an earlier datafile. Then it
> worked. Is it ok to do this?
>
> I did step 3 as well, using the field individual (which you get by
> keepindividuals = 'yes'). In step 4, I should just use cfg.statistic =
> 'depsamplesT', right (because the variables are within subject)?
>
> Thank you!
> Sara
>
> On 25/11/2011 09:04, Stolk, A. wrote:
> > Hi Sara,
> >
> > If I understand correctly, you want to test intra-subject
> differences (between conditions) at the second level? This would
> require the following steps:
> >
> > 1) subject-level statistics, which you have done already
> >
> > 2) grandaverage all these, with keepindividuals=yes.
> >
> > 3) copy the output of the grandaverage (into a dummy variable), and
> replace the fields containing the subject T-values with zeros (for
> timelock data this may be the trial fields?)
> >
> > 4) again timelockstatistics, as in step 1, now with the variables
> following step 3. this should give you the resulting statistics of
> contrasting intra-subject differences vs. null at the group level.
> >
> > Hope this helps,
> >
> > Arjen
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> >
> > ----- "Sara Bögels"<s.bogels at psy.gla.ac.uk> schreef:
> >
> >> Van: "Sara Bögels"<s.bogels at psy.gla.ac.uk>
> >> Aan: fieldtrip at donders.ru.nl
> >> Verzonden: Donderdag 24 november 2011 15:49:17
> >> Onderwerp: [FieldTrip] second-level statistical inference
> >>
> >> Hi all,
> >>
> >> I have been trying to do second-level statistical inference (as
> >> described in one of the FAQs) on ERFs, but I am not sure whether I
> am
> >>
> >> doing everything correctly.
> >>
> >> In the first step I calculate the T-values for the difference
> between
> >>
> >> two conditions (twice), which are between items, with
> >> ft_timelockstatistics. I put the output of all participants in a
> cell
> >>
> >> (called 'stat1a' and 'stat1b'). (I tried to use
> >> ft_timelockgrandaverage
> >> to combine the subjects together but it needs a field avg).
> >>
> >> Then I use ft_timelockstatistics again but subject level. I first
> >> want
> >> to look at the difference between the two conditions. This
> difference
> >> is
> >> reflected in the T-values of the first step so I create a dummy
> which
> >> is
> >> the same as 'stat1' but I replace all the values in the field
> 'stat'
> >> per
> >> participant with zeros. Then I call (with appropriate cfg
> >> parameters):
> >>
> >> stat2a = ft_timelockstatistics(cfg,stat1a{:},dummy{:});
> >> stat2b = ft_timelockstatistics(cfg,stat1b{:},dummy{:});
> >>
> >> To compare the two differences (stat1a and stat1b) and thereby look
> at
> >>
> >> an interaction, I call:
> >>
> >> stat2a-b = ft_timelockstatistics(cfg,stat1a{:},stat1b{:});
> >>
> >> I am uncertain whether the dummy works (or is there a way to
> compare
> >> the
> >> t-values to zero directly?) and whether the stat1a{:} trick works
> with
> >>
> >> ft_timelockstatistics.
> >>
> >> Thanks in advance for your answer.
> >>
> >> Sara
> >>
> >>
> >> _______________________________________________
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> >> fieldtrip at donders.ru.nl
> >> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
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