[Eeglablist] statistics in EEGLAB
Arnaud Delorme
arno at ucsd.edu
Wed Nov 11 13:10:10 PST 2009
Dear Bob,
thanks for the comments. I think you are using the statmode option
"trial" from the command line. This option is quite experimental. It
was implemented a while ago and is probably not forward compatible
with more recent changes. Also, the "statmode", "trials" option
(assuming it was working) should only be used to plot a single
subjects. The reason is based on the type of null hypothesis.
When testing with 'statmode', 'subject' for two conditions, the NULL
hypothesis is: given the subjects I have recorded and given that these
subjects are a good representation of the general population of all
possible subjects, there is no difference between the ERP/spectrum/
ERSP/ITC between the two experimental conditions in the general
subject population. Using parametric, permutation, or bootstrap
statistics (and assumptions) you may either accept or reject this
hypothesis at a given confidence level.
When testing with 'statmode', 'trial' on a single subject (still two
conditions), the NULL hypothesis is : given the trials I have recorded
and given that these trials are a good representation of all the
population of trials for this subject, there is no difference between
the ERP/spectrum/ERSP/ITC between the two experimental conditions for
this subject. Again, using parametric, permutation, or bootstrap
statistics (and assumptions) you may either accept or reject this
hypothesis at a given confidence level.
As you can see the two hypothesis are quite different. One makes an
inference about the population of subjects and the other one about the
population of trials.
Now if you pool the trials from different subjects and attempt to
perform statistics, this is going to be more complex. The new
hypothesis would then be: given the trials I have recorded from my
subjects and given that these trials are a good representation of all
the population of trials from the general population of subjects,
there is no difference between the ERP/spectrum/ERSP/ITC between the
two experimental conditions in the general population of subjects. But
the hypothesis is relatively biased because I personally think that
all the trials are not a good representation of all the population of
trials from the general population of subjects. The trials are a good
representation of all the trials from all the subjects being presently
recorded but not necessarily of the general subject population.
Therefore the real NULL hypothesis would be : given the trials I have
recorded from all of my subjects and given that these trials are a
good representation of all the population of trials from these
subjects, there is no difference between the ERP/spectrum/ERSP/ITC
between the two experimental conditions in the recorded subjects. As
you see, rejecting the NULL this is relatively limited as we care
about the general population of subjects and not the recorded subjects.
If anybody has some better ideas (or Matlab function) of how to handle
the subject/trial problem (because it would be nice to include trials
in statistical analysis in order to make them more powerful), we will
take them.
Best,
Arno
ps: we will remove the 'statmode', 'trial' option for now.
pps: for basic inferential statistics, you may also refer to this book
chapter http://sccn.ucsd.edu/~arno/mypapers/statistics.pdf
On Nov 11, 2009, at 12:29 AM, Robert Brown wrote:
> Dear Arno & others,
>
> this does not seem to be as simple as Arno suggested (but thanks),
>
> 1. I have precomputed the values of these channels (with
> "savetrials", "on")
> 2. these channels all have data
> 3. I can plot the data of the same channels when I use "statmode",
> "subjects"
> 4. I'm using EEGLAB v7.1.3.13b
> 5. I now tried it with v7.1.7.18b and I still get the log of zero
> error (you guys might be interested that in addition I now get, in
> case of permutations and bootstrap, "??? Error using ==> reshape" in
> statcond>surrogate at 438 and statcond at 301 and with this latest
> version the reshape error even happens with the "statmode",
> "subjects")
>
> thus any other suggestions of what could be happening with my single
> trial analysis in study would be very much appreciated.
>
> thank you very much and take care,
> Bob
>
> 2009/11/11 Arnaud Delorme <arno at ucsd.edu>
> Dear Bob,
>
> I think this might be because you are trying to plot ERSP of a
> channel that contains only 0. This error was also arising in old
> versions of EEGLAB when masking for significance.
>
> Hope this helps,
>
> Arno
>
>
> On Nov 7, 2009, at 11:38 AM, Robert Brown wrote:
>
> Hi guys,
>
> I've been trying to get the study ersp analysis working on single
> trials but I've not succeeded.
>
> in the function "std_readdata" I get the "Warning: Log of zero."
> error, which is on the line ersp{c,g} = 20*log10(abs(ersp{c,g}));
> meaning that the absolute value at some point is 0.
> (This leads to) further errors:
>
> ??? Error using ==> set
> Bad value for axes property: 'CLim'
> Values must be increasing and non-NaN.
>
> Error in ==> caxis at 80
> set(ax,'CLim',arg);
>
> Error in ==> tftopo at 714
> caxis([g.limits(5:6)]);
>
> I've tried to fix it but I'm not clever enough. any help would be
> appreciated.
>
> thank you so much,,
> Bob <ATT00001.txt>
>
>
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