Earlier quoted context omitted.
I just needed to stop and say: as a biostatistician, boy do I love a beautiful complex longitudinal design: I remember my old professor asking us how at this point we would decompose into cross-sectional and longitudinal effects, Lord's paradox, etc... and I still don't fully understand Lord's paradox as well as I should.
This is a very important idea. For example, one issue with accelerated longitudinal designs, see image [1], is that while they efficiently cover a larger age range, the fixed effects of age are largely driven by cross-sectional differences between who is samples are younger and older ages. One method that can be used to test whether the pattern seen in the fixed effects represents the pattern within subjects is to de…
[1] https://onlinelibrary.wiley.com/doi/book/10.1002/97811195134...