Well, unless you think there might be different effects across sessions 
(e.g. different response profiles in late vs. early sessions), it 
probably doesn't matter. I would still just model it how the subject saw it.

Elise Temple wrote:
> Yes,  your understanding is correct - and for each subject it was a
> different order of runs.  I agree that it is not difficult to model this in
> the batch system  - which is why I wanted to do it that way.  The question
> is - does it matter if the sessions aren't modeled int he way they actually
> occurred.  Donald's email suggests it doesn't matter.  The only I can think
> of that it could is if there are any assumptions about those regressors -
> that the order does matter - e.g., scanner drift that occurs over time would
> be accounted for.  My thought was if SPM treated these regressors as ordinal
> numbers that are related in time it would be better to reflect that in the
> order they were entered into the design.  If, however, they are treated as
> nominal categories that don't actually ahve a relationship in time then it
> shouldn't matter.
>
> elise
>
> On Wed, 26 Aug 2009 17:21:02 -0400, Chris Watson
> <[log in to unmask]> wrote:
>
>   
>> Perhaps I'm not understanding something here, but are you saying that,
>> you modeled every subject as having seen e.g. lists 1-2-3-4-5-6, even if
>> they saw, e.g. 6-5-4-3-2-1?
>> With SPM8's batch system and even before, it shouldn't be an
>> inconvenience to set up different models for different subjects.
>>
>> Elise Temple wrote:
>>     
>>> I am having a debate with one of my graduate students.  We have 6 runs
>>> (sessions) in this experiment.  We have 6 different lists with the
>>> conditions in different orders in each list.  Because of counterbalancing
>>> the order the lists is different across subjects (some do list 1 1st, some
>>> list 2, etc).   While doing single subject 1st level analysis, for pure
>>> convenience reasons, she did not model the sessions in the order they
>>> actually occurred, but rather based on the list they were doing.
>>>
>>> My thought is that the session effects not only represent the variance that
>>> is different between each session, but should also represent the actual
>>> order the subject did the sessions because there is meaning in the order of
>>> them.  The question is - does the way SPM deals with this regressor take
>>> into account sessions as an ordinal value where their number has meaning or
>>> does it treat them as nominal so it really doesnt matter what order they are
>>> put in?
>>>
>>> It still seems to me one's model should match as closely as possible what
>>> you think actually is going on - but if its purely for aesthetic reasons she
>>> probably doesnt have to reanalyze.   On the other hand, if the order is
>>> meaningful and by shuffling them we are not taking advantage of how
>>> effective the estimate of these regressors are then reanalysis is necessary
>>> even if no visible differences are seen between the results.
>>>
>>> BTW: We are using SPM8.
>>>
>>> best,
>>>
>>> elise temple
>>>
>>>
>>> Elise Temple, Ph.D.
>>> Educational Neuroscience Laboratory
>>> http://www.dartmouth.edu/~templelab/
>>>
>>> Assistant Professor
>>> Department of Education
>>> Graduate faculty in Psychological & Brain Sciences
>>> Dartmouth College
>>> Hanover, NH 03755
>>>
>>>
>>>       
>
>
>
>