I have a study design with a between subjects factor (treatment: verum vs placebo) and a within subjects factor (time: before vs after). Subjects were entered into the model as a random effect.
So far this is straightforward to implement, however now a third factor comes into play. The subjects were recruited in two "waves", so the this factor can be named seasonality with two levels (winter vs spring). I am not sure how this factor can be introduced into the mixed model. Because I'm not interested to estimate differences in the levels of seasonality I would think this can be considered as an additional random effect? On the other side seasonality has only two levels and so it doesn't seem to make sense to consider it as a random effect.