# When no model comparison, should I use REML vs ML?

I'm running LMM, and I will make no comparison of models. Could I ask which one should I use between REML and ML?

• When there is no model comparison, the difference between restricted (or residual) maximum likelihood (REML) and maximum likelihood (ML) is that, REML can give you unbiased estimates of the variance parameters. Recap that, ML estimates for variance has a term $1/n$, but the unbiased estimate should be $1/(n-p)$, where $n$ is the sample size, $p$ is the number of mean parameters. So REML should be used when you are interested in variance estimates and $n$ is not big enough as compared to $p$.