I´m still new to statistical modeling and quite confused about how I should correctly specify my model.
I have 3 crossed categorical treatments - stress, infection and food. Each individual was measured at 4 time points (just before the experiment and 3x during the experiment).
As there are only 4 time points and I have no reason to assume that changes in response variables will change in a single direction through time (as growth would), I treated time as a categorical variable. So the full model would be:
I hope so far, everything´s right.
But I don´t seem to understand if I need a random slope too.
I tried to plot response~time for individuals of each experimental group and noticed a big difference in shape of response between them. So I´d say there is much noise still to be filtered. Does it make sense then to put time as a random slope? Anyway I used it as a fixed categorical variable, so I suppose it shouldn´t be specified as continuous in random part of the model then? But what does it tell me random categorical slope? Does it even make any sense?
I´m sorry if the question is stupid, and thank you very much for your answers.