For mixed model, when we estimate variance component by restricted maximum likelihood estimation procedure, an iterative algorithm is required to solve the estimating equations for variance component. Two main iterative algorithms are :
(1) Expectation-Maximization Algorithm ;
(2) Newton-Raphson Algorithm .
I am using
lmer to estimate parameters of mixed model by restricted maximum likelihood estimation method. But I don't know which iterative algorithm it uses for solving the estimating equations for variance components. Does it use (1) or (2) or other iterative algorithm ?