Observations are taken from a mixture of several populations and the problem is to estimate the unknown mixing proportions on the basis of this sample which is unclassified; that is, it is not known to which population an observation belongs. There are reference data of known origin available separately from each of the populations for the purposes of estimation of the unknown population parameters. Two approaches to this problem are illustrated by the presentation of a case study involving an unclassified sample of n = 1107 four-dimensional observations taken from a mixture of seven populations. The usefulness of atypicality indices for assessing whether an unclassified observation comes from one of the specified populations is exhibited.