  A large number of explicit estimators are proposed in this paper for loss
rate estimation in a network of the tree topology. All of the estimators are
proved to be unbiased and consistent instead of asymptotic unbiased as that
obtained in [1] for a specific estimator. In addition, a set of formulae are
derived for the variances of various maximum likelihood estimators that unveil
the connection between the path of interest and the subtrees connecting the
path to observers. Using the formulae, we are able to not only rank the
estimators proposed so far, including those proposed in this paper, but also
identify the errors made in previous works. More importantly, using the
formulae we can easily identify the most efficient explicit estimator from a
pool that makes model selection feasible in loss tomography
