I've got some problem:
Check the relationship between income tax income and migration in Polish "powiats" (an administrative unit in Poland, something like a county in the US). I have data in this regard for individual years. I think a good model would be:
Tax_income ~ balance_of_migration + year + (1|powiat).
However, I am not sure about it because of one thing. I know that random effects should be used, when we have a factor with levels that have been drawn from a population of possible samples.The situation is different here. I have all counties. However, my intuition tells me that my model is good for my problem, because I want to take into account the fact of repeated observations for powiats, but I am not interested what is the change in the values of the endogenous variable for individual powiats (however, it will be a good idea to assess the variability (variance) between counties).
What do you think: can I use 'powiats' as an random effect in model?