A recommendation: talk to a statistician about implementing an analysis for missing data. There are a number of options that may be available to you to allow a more straightforward and scientifically sound approach.

Anthony

Dear FreeSurfer users

I conducted a longitudinal study with 3 timepoints: baseline, 8 weeks
and 24 weeks.  Between baseline and 8 weeks participants were randomized
to one of 3 training programs. I want to assess changes in gray matter
volume from baseline to 8 weeks, and baseline to 24 weeks, and also
determine how these changes correlate with various behavioral outcome
measures. The correlations will be conducted in SPSS, so I need to
export values from each time point to enter into the regression matrix.

The issue is that about 20 people dropped out after the 8 week
assessment.  So to just ask the question about changes in brain
structure I created two different processing streams, one for all 75
subjects who have pre and 8 week data, and a second one for the 58
individuals who have pre and 24 week data. Therefore, participants who
have data for all 3 time points had their pre and 8-week data analyzed in both
processing streams. These separate processing streams yield different
pre values for these individuals (e.g. the hippocampal volume of
participant X at “pre” in the first processing stream is different than
the pre hippocampal volume of the same participant in the second
processing stream).

My question is about the proper way to combine data from these two
processing streams so that I have one Excel column for each of the 3
time points to be analyzed in SPSS with all participants included. One
approach would be to use the values from the second processing stream
(which includes the 24 week time point) for the participants who have all
3 time points, and use  the data from the first processing stream only for
those who are missing the 24 week time point. But I am not sure whether this is acceptable given the discrepancy in values between the two processing streams. So, is there better way to combine the data from both streams into these 3 time-point columns?

Thank you

Jonathan

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-- 
Anthony Steven Dick, Ph.D.  
Associate Professor
Director, Cognitive Neuroscience Program and Graduate Certificate in Cognitive Neuroscience  
Department of Psychology  
Florida International University Modesto A. Maidique Campus AHC4 454  
11200 S.W. 8th Street  
Miami, FL 33199  
Ph: 305-348-4202; Lab Ph: 305-348-9055; Fx: 305-348-3879  
Email: adick@fiu.edu
Webpage: http://myweb.fiu.edu/adick
Join the Society for the Study of Human Development: http://www.sshdonline.org