This estimate will have to be determined likely by utilizing sampling in some form. Since such an estimate will come from partial information (i.e. a sample) then it is very likely going to only be an approximation to the exact (but unknown) value. In general, an estimator is a numerical value which is used in the place of an unknown population statistic. To determine precisely what is a "best" estimator requires a multivariate approach and is beyond the scope of this text. Indeed, to justify why each of the following are good estimators look up the topic "Maximum Likelihood Estimators".