Dear all,

I collected data from subjects in a longitudinal study. However, I now want to compare groups of persons in a cross-sectional fashion. My problem is that some subjects appear twice in the same group, because they were tested twice (1-year gap), while others appear only once in the group. While I am sure that it's problematic to analyse a subject twice if they were tested at the same time, I am not sure whether or why it is justifiable to analyse the person twice within the same group when he or she was tested at two different times.

Thanks for your help,

Johannes

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