[FieldTrip] Excluding a subject

Roni Tibon ronitibon at gmail.com
Thu May 10 20:55:53 CEST 2012

Dear Stephen and Steve,

Thank you for your answers.

Stephen, if I understand you correctly, option 1 means I have to compute
the grand average again (using ft_timelockgrandaverage), which is what I
tried to avoid.

I tried option 2 though, and it didn't work.
When I used a zero entry at the uvar line, the data stayed exactly the same
(as if the subject was not excluded). When I used a zero entry at the ivar
line (or at both lines), i get an error message saying "invalid
specification for the design array".

I also tried skipping the entry all together, but got an error message
saying "the size of the design matrix does not match the number of
observations in the data"

An I doing something wrong?

Steve, I guess I can do that.. the reason I'm asking is cause I want to
"play" with the data a bit - remove a different subjects each time and see
if I still get the same pattern. So I thought it would be easier if I did
not have to compute the grand average again and again.


On 10 May 2012 14:07, Stephen Whitmarsh <stephen.whitmarsh at gmail.com> wrote:

> Hi Tony,
> Thanks for your question.
> First of all, I do not fully understand what you mean with 'without
> computing the averages again', but I'll have a shot at it.
> In ft_freq/timelock-statistics you enter all your 'units of
> observations', i.e. data structures per subject or condition that you
> want to compare, together with the design matrix. The designmatrix is
> a one or two row matrix with as many columns as your have
> data-entries. One of the rows codes the group/condition membership of
> those entries (the independent variable). They can be subjects (group
> analysis) or trials (within-subjects analysis). Another row might be
> used to code the observation number in case you are doing a paired
> test.
> To exclude one data entry (i.e. subject) for your
> ft_timelockstatistics you could either:
> 1) not enter that datastructure as data in ft_freqstatistics, and also
> remove it from your design matrix.
> 2) keep your input to ft_timelockstatistics the same but use a zero
> for that entry in you design matrix. In this case that data input will
> not be used to calculate your statistics.
> I would say the second option is more elegant, but it is more prone to
> mistakes perhaps. You would do me a favour if you would doublecheck
> that it gives you the same results as option 1.
> I hope this answers your question,
> Stephen
> On 10 May 2012 12:09, Roni Tibon <ronitibon at gmail.com> wrote:
> > Hi all,
> > Is there an elegant way to exclude a subject from the statistical
> analysis
> > when using ft_timelockstatistics, without computing the averages again
> while
> > excluding the subject?
> >
> > Thanks!
> > Roni
> >
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