  Theoretical predictions of physical observables often involve extrapolations
to regions that are poorly constrained by laboratory experiments and
astrophysical observations. Without properly quantified theoretical errors,
such model predictions are of very limited utility. In this contribution we use
maximum-likelihood estimation to compute theoretical errors and assess
correlations between physical observables. We illustrate the power and elegance
of these methods using examples of both pedagogical and realistic interest. In
particular, we implement a gaussian approximation to the likelihood function to
develop a new relativistic effective interaction constrained by ground-state
properties of finite nuclei, their monopole response, and masses of neutron
stars.
