I am analyzing data on "BloodSugar" level (dependent variable) and trying to find its relation with "age", "gender" and "weight" (independent variables) of subjects. I have data collected from subjects sampled in four "city".
Should I use "city" variable as fixed effect or a random effect?
So which is correct:
lm(bloodsugar ~ age + gender + weight + city, mydata)
lmer(bloodsugar ~ age + gender + weight + (1|city), mydata)
Thanks for your help.
Edit: In response to comment by @Dave , I would like to add following: Currently there is no data on relation between my real dependent variable and City. So, relation could be there. Relation with City is not my primary objective but it will be nice to determine that relation also, if it is feasible by proper statistical methods.