Atypical Repeated Measures analysis

Question

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.

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.