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Do random effects have to be categorical?
The random effects aren’t hard to see: Those are μ0 the random intercept, and μ1 the random slope over time. There is also a random factor here: County. We use the term “random factor” and not “random variable” because random variables in a mixed model MUST be categorical.
Can a variable be both fixed and random effect?
From the information you have given, I would say its a fixed effect, however, a variable can be fixed and a random in the same model. the fixed effect in these cases are estimating the population level coefficients, while the random effects can account for individual differences in response to an effect.
How many levels does a random effect have?
Short answer: Yes, you can use ID as random effect with 6 levels.
What do you need to know about random effects?
Software programs do provide access to the random effects (best linear unbiased predictors, or BLUPs) associated with each of the random subjects. BLUPs are the differences between the intercept for each random subject and the overall intercept (or slope for each random subject and the overall slope).
How is a random effect associated with a categorical variable?
A random effect is always associated with a categorical variable. This categorical variable will most often divide the observations into different observational units (this could for instance be Dam in your data set as it seems reasonable to assume that observations from the same dam are more alike than from different dams.
Why are there random effects in mixed models?
In addition to students, there may be random variability from the teachers of those students. Some doctors’ patients may have a greater probability of recovery, and others may have a lower probability, even after we have accounted for the doctors’ experience and other measurable traits.
What do random slopes and random intercepts mean?
Random intercepts allow the outcome to be higher or lower for each doctor or teacher; random slopes allow fixed effects to vary for each doctor or teacher. What do these random effects mean? How do we interpret them? We usually talk about them in terms of their variability, instead of focusing on them individually.