[FieldTrip] second-level statistical inference

Sara Bögels s.bogels at psy.gla.ac.uk
Thu Nov 24 15:49:17 CET 2011

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 

Thanks in advance for your answer.


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