Hi,
I think it may actually be inherent in spm_realign: if you look at lines
179ff, it seems that the grid that is initiated for sampling is then
slightly modified using random numbers. I dimly remember John mentioned
something about the reason for that on the list or in private years ago,
but I can't seem to find it. Probably was a good one, though :) So
bottomline, I believe that the points the images are sampled at are
meant to very slightly differ from run to run, so it is not suprising
the results would, too. In any case, differences on the order of
magnituded you mentioned (10^-9) would not worry me too much.
Cheers,
Marko
S.F.W. Neggers wrote:
> perhaps because when you realign already realigned images again,
> corrections will get smaller every time you do this?
>
> Good luck,
>
> Bas
>
> Op 17-10-11 10:13, Natalia Yakunina schreef:
>> Hello everybody
>>
>>
>> I was wondering if it's normal that after I run realignment several
>> times with EXACTLY the same set of parameters, motion parameters
>> rp*.txt files come out slightly different EVERY TIME (not all of the
>> values are different, but some of them are; the difference is of order
>> 10^-9 but still..). I've run it five times, and all five sets of files
>> are different from each other.
>> If this is how it is supposed to be, what's the reason for that? And
>> how much does this kind of difference affect GLM results?
>> I am trying to reproduce some old results, and I've stumbled on this
>> the very first step.
>> Thank you!
>>
>>
>> Best Regards,
>> Natalia
>
>
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____________________________________________________
PD Dr. med. Marko Wilke
Facharzt für Kinder- und Jugendmedizin
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Universitäts-Kinderklinik
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