[FieldTrip] ft_timelockstatistics tails

[Bianca Trovò]

May I ask a related question? In this case (cfg.correcttail = 'prob'), am I
comparing the *two-tailed p-value of stat.prob* with *0.05* of cfg.alpha or
with *0.025* from cfg.correcttail = ‘alpha’?
So in the example above ( where actual two-tailed p-value is 0.0376), am I
able to reject or not the Null-Hypothesis?

Many thanks,
Bianca

Il giorno mar 23 apr 2019 alle ore 18:56 Mathis Kaiser <
mathis.kaiser at charite.de> ha scritto:

> Hi Patrick,
>
> unless you have specified cfg.correcttail = 'prob', you are correct in
> assuming you have to double the cluster-level p-values for a two-tailed
> test.
>
> See
>
> http://www.fieldtriptoolbox.org/faq/why_should_i_use_the_cfg.correcttail_option_when_using_statistics_montecarlo/
>
> Best,
> Mathis
>
> On 23.04.19 17:18, psdwyer at ucdavis.edu wrote:
> > Hello,
> >
> > I was wondering if I could ask a clarification question about the
> > correct interpretation of the outputs of Fieldtrip’s
> > ft_timelockstatistics function?
> >
> > Online, it says:
> >
> > “We use cfg.alpha to control the false alarm rate of the permutation
> > test (the probability of falsely rejecting the null hypothesis). The
> > value of cfg.alpha determines the critical values with which we must
> > compare the test statistic (i.e., the maximum and the minimum
> > cluster-level statistic). Note that if you want to run a two-sided test,
> > you have to split the critical alpha value by setting cfg.correcttail =
> > ‘alpha’; i.e. this sets cfg.alpha = 0.025, corresponding to a false
> > alarm rate of 0.05 in a two-sided test. The field cfg.alpha is not
> > crucial. This is because the output of ft_timelockstatistics (see
> > further) contains a p-value for every cluster (calculated under the
> > permutation distribution of the maximum/minimum cluster-level
> > statistic). Instead of the critical values, we can also use these
> > p-values to determine the significance of the clusters.”
> >
> > I interpreted this to mean that I should double all the p-values I get
> > in the final output, since I am running two-tailed tests. E.g., if I
> > specify that I want a two-tailed test in cfg.tail and if I then get an
> > output saying:
> >
> > stat.posclusters
> > ans =
> > struct with fields:
> > prob: 0.0188
> > clusterstat: 160.9688
> > stddev: 0.0014
> > cirange: 0.0027
> >
> > Then I interpret it to mean that the actual two-tailed p-value is
> > 0.0376, not 0.0188. However, I’m having second thoughts about this, and
> > wondering if I have this right.
> >
> > I’m afraid this is kind of urgent – I’m supposed to submit slides
> > tomorrow for a talk I’m giving where we analyze data using this
> > function, so if you have a chance to confirm whether I am interpreting
> > this correctly or not, that would be super-appreciated! Thanks so much,
> >
> > Patrick
> >
> >
> >
> > Patrick Dwyer
> >
> > Ph.D. Student, Developmental Psychology
> >
> > Neurocognitive Development Lab <http://riveralab.ucdavis.edu/>
> >
> > Center for Mind and Brain, UC Davis
> >
> >
> >
> > Personal Blog: http://www.autisticscholar.com/
> >
> > Pronouns: he/him/his
> >
> >
> > _______________________________________________
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> > https://doi.org/10.1371/journal.pcbi.1002202
> >
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>


-- 
*Bianca Trovò*

PhD student, Sorbonne Université
Cognitive Neuroimaging unit, INSERM U992
Neurospin center, CEA/SAC/DSV/I2BM
Bât 145,Point Courier 156, 91191 Gif-sur-Yvette Cedex, FRANCE
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Phone (office): +33169089319
Email (office): *bianca.trovo at cea.fr <bianca.trovo at cea.fr>*
*Unicog Lab - Computational Brain Team
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