I am running an analysis with longitudinal study design. I have previously described data my data here. In general, why is time considered as a numeric variable rather than a factor in a longitudinal analysis?
This might seem obvious, but time is a continuous variable, thereby it is a numeric variable. Factors are normally used when we have group variables, as
sex. We can't really measure the "difference" between
female, but we know what means a difference of one unit of time.