 
Summary: J. theor. Biol. (1995) 177, 247262
00225193/95/230247+16 $12.00/0 7 1995 Academic Press Limited
Discrimination Between Two Species of Microtus using both
Classified and Unclassified Observations
J.P. AIROLDI, B. D. FLURY AND M. SALVIONI§
BES, Biology, University of Bern, Bern, Switzerland,
Department of Mathematics, Indiana University, Bloomington, IN 47405, USA and
§ Dipartimento dell' Ambiente, Ufficio Caccia e Pesca, Bellinzona, Switzerland
(Received on 21 November 1994, Accepted on 19 June 1995)
We analyse a set of morphometric data obtained from the skulls of 288 specimens of Microtus
subterraneus and M. multiplex. The chromosomes of 89 specimens were analyzed to identify the species;
species is unknown for the remaining 199 specimens. In this situation one may either use the classified
observations to estimate a discriminant function (this is the traditional approach of discriminant
analysis), or one may attempt to use the unclassified observations as well to improve parameter
estimation. The latter case, which we refer to as ``Discrimix'', is a combination of discriminant analysis
and finite mixture analysis which appears to be essentially unknown among biologists. Yet the method,
the statistical theory of which is fairly well developed under the name ``discriminant analysis with
partially classified data'', has the potential to greatly improve the estimation of classification rules, as
we illustrate using the Microtus data. Like finite mixture analysis, Discrimix requires iterative
computations to estimate the parameters, but has the advantage of fully using the information contained
