Bioequivalence and Bioavailability Forum

Hello everyone,

Recently, we performed a 4-treatment (T1, T2, T3 and T4), 4-period, cross-over study pharmacokinetic study.

Once the statistical analysis performed, I observed that for the comparison between T1 and T3 and between T2 and T4, the number of degrees of freedom for the fixed effect "period" is 2 instead of 3 (which is the case for the other comparisons). Furthermore, "Not estimable" is mentioned in the output of Winonlin.

For information, the statistical analysis was performed using ANOVA model (or equivalent parametric method) with fixed effects usually sequence, subject within sequence, period and formulation.

To be honest, I don't know how to fix it and I don't know how to interpret this.
Could you please inform me if it is an issue related to the software (Winonlin) or is it related to the model used?
Additionally, could you explain me how to interpret these results and what should be the impact on the results obtained form the analysis?

Thanks for your help