I am estimating a Multilevel Mixed Effects model in stata, likelihood ratio tests imply that this model is preferred over a normal OLS model.

Additionally, I suspected my data set to be subject of sample selection bias on an observable variable and therefore performed a Heckman test. The results indicate significant sample selection bias and hence that I should include the Mills ratio estimated by the Heckman test.

My question is if I can now just include the Mills ratio in my Multilevel Mixed Effects model. I can find very little information on correcting for Sample selection bias in Multilevel Mixed Effect models and wonder if a Heckman correction can be used in a Multilevel Mixed Effects model at all as it is normally used in OLS models.

Thanks for your help in advance,

Fabrizio