+ In the setting \code{method = "ML"} and \code{model = "BM"} (this used
+ to be the default until \pkg{ape} 3.0-7) the maximum likelihood
+ estimation is done simultaneously on the ancestral values and the
+ variance of the Brownian motion process; these estimates are then used
+ to compute the confidence intervals in the standard way. The REML
+ method first estimates the ancestral value at the root (aka, the
+ phylogenetic mean), then the variance of the Brownian motion process
+ is estimated by optimizing the residual log-likelihood. The ancestral
+ values are finally inferred from the likelihood function giving these
+ two parameters. If \code{method = "pic"} or \code{"GLS"}, the
+ confidence intervals are computed using the expected variances under
+ the model, so they depend only on the tree.
+
+ It could be shown that, with a continous character, REML results in
+ unbiased estimates of the variance of the Brownian motion process
+ while ML gives a downward bias. Therefore the former is recommanded.
+