[Eeglablist] ask again about bootstrap and group analysis. please help

[Christopher Summerfield]

dear wu xiang

I think the procedure is outlined quite clearly in the Burgess & Gruzelier
article Phil Grieve just sent you, but to explain step by step...

1) you have coherence estimates at p electrode pairs for s subjects in 2
conditions.  This means that you can perform a t-test at each electrode.
If you have several estimates per pair, for example several time bins, or
perhaps even an entire time-frequency map, you may have several
t-estimates per pair. I will call these 'point estimate' statistics.

2) at each pair, take your subject-condition assignments and swap them
1000 (or more times), recomputing your t-values at every pair or at every
timepoint or whatever.  This will give you a null distribution
corresponding to every point estimate stat.

3) you can then determine whether each point estimate falls within alpha
of its null distribution.

4) to correct for multiple comparisons (for example, imagine that you have
20 electrodes=190 pairings), rather than checking each point estimate
against its distribution, enter (for each permutation) the maximum
statistic across your entire search space. So, in our example, if the
maximum permuted value from permutation 1 comes from electrode 188, you
enter that value into the null distribtuion. on the 2nd permutation, the
max value might come from another electrode.  You then check all your
electrodes against this distribution. This will provide an 'familywise
error' correction for multiple comparisons.

good luck
Chris