Repeated measures correlation


I follow a group of participants and I take different measurements over time. For instance, throughout a year I assess my participants on 4 different occasions, hence, we have 4 different time points.

I would like to check how the measures for a specific parameter correlate with another one. For instance, I have been taking blood samples and I would like to see whether the levels of a protein called BDNF in plasma correlate with the levels from serum.

But my doubt is: How can I check whether plasma measures across 4-time points correlate with serum measures across 4-time points?

Do I have to check individually time1_plasma vs time1_serum, time2_plasma vs time2_serum, etc? I think there should be a way to do it together but I am not sure.

Any help would be greatly appreciated.

Many thanks,


TS Contributor
In the olden days, in such a situation I just performed intra-individual correlations
and then aggregated the coefficients (for example, by using the median).

With kind regards