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I have performed a mixed-design ANCOVA on cortisol values (in SPSS), with 'time' as the within-subject variable, 'group' as the between-subject variable and 'overall treatment time' as a covariance variable. I found a significant interaction between 'time' and 'group' which I could interpret using contrasts (planned comparisons), indicating there is a interaction such that cortisol values in the experimental group increase significantly more than the control group between two time-points (pre- and post-intervention). This is a great result, however, I would also like to know wether this increase in cortisol values in the experimental group is a significant increase on itself (i.e. when only considering the experimental group, is the increase between these time-points significant?). This information (as far as I understand) can not be obtained from the information in the mixed-design ANCOVA. I therefore figured that to gain this insight, an additional ANCOVA is needed, using only data from the experimental group: with 'time' as the within-subject variable and 'overall treatment time' as a covariance variable.

My question is: am I correct in assuming this information cannot be obtained from the mixed-design ANCOVA, and if so, should I correct the obtained significance from the second test, and if so, how?

Thanks a lot in advance!

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