I have a research of 30 patients (one sample) which performed cognitive tests in the following states:

  1. Baseline
  2. at 3 months with no intervention
  3. at 6 months after intervention

These are the same patients who underwent all 3 states.

The cognitive measures are continuous parameters and I am using SPSS. I am wondering how to analyze them:

  1. Paired t test of 1-2 compared to 2-3
  2. Repeated measures with 1 as covariate ? is this "legal"?
  3. splitting as 2 groups of 1-2 and 2-3 so each has a pre score and a post score and performing a ANOCVA with the pre score as covariate (i.e. for 1-2 group the covariate values will be 1, and for 2-3 group the covriate values will be 2)

I realize that this is an atypical cohort, but any help will be greatly appreciated.

  • $\begingroup$ What specific question do you want to investigate? $\endgroup$
    – Michael M
    Apr 29, 2017 at 17:24
  • $\begingroup$ Thanks for the reply. I want to investigate if the cognitive functions improved after the intervention or not. The first and second points were made for test-retest validation and removing a possible "learning" effect. $\endgroup$
    – XPeriment
    Apr 29, 2017 at 21:55
  • $\begingroup$ help anyone? ? ? $\endgroup$
    – XPeriment
    May 2, 2017 at 18:30

1 Answer 1


Your design is clearly worse than a pre/post design with randomization into control and test and clearly better than an intervention only pre/post design.

In case of no (time-dependent) confounders I would favour suggestion 1) as it is simple and easy to explain to others. I guess you are aware that it assumes constant placebo effect over time. To avoid relying on normality assumptions, maybe go for a permutation t test. This should be possible even in SPSS.

Option 2) could be interesting in case you want to take into account (time-dependent) confounders.


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