Hi Nicola,
It has been my empirical experience that you will indeed get exactly the
same result (i.e., same permutations) if you run randomise using the
identical command a 2nd time. (Use the -P flag to save the actual
permutation vector if you are interested in comparing the permutations
employed across repeated randomise instantiations). However, note that
if you have multiple contrasts specified in a .con file, the
permutations that you get for the 2nd, 3rd, etc contrasts are NOT the
same as you would get if you ran those contrasts as the only contrast in
the .con file. That is, within a given instance of randomise, its
internal random seed does not appear to "reset" to the initial value for
consecutive contrasts. (This would IMHO be a useful feature to have
available as an optional flag -- for example if one wishes to run both
positive and negative contrasts using the identical set of permutations
within a single run of randomise).
Don't know how this all applies with randomise_parallel...
cheers,
-MH
On Thu, 2011-08-04 at 14:47 +0200, Nicola Toschi wrote:
> Dear all,
>
> I was wondering how the seeding is done in randomize.
>
> In other words, if I run the same randomise command multiple times
> (assuming the number of permutations is lower that the number of all
> possible permutations), will I always get exactly the same result? Does
> the same answer hold for randomise_parallel?
>
> Thanks in advance,
>
> Nicola
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