[FieldTrip] ft_timelockstatistics tails

[psdwyer at ucdavis.edu]

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