The use of the Coefficient of Difference (CD), which was first suggested some 50 years ago, as a measure of difference of a single character between two populations is not recommended today on grounds of statisticial weakness, its inability to evaluate more than one character simultaneously, and the often discordant variation of the characters studied. It is useful to utilize the statistically proven t-test or analysis of variance in cases when a few characters are studied, while multivariate distance measures such as Euclidian distance or discriminant function analysis will help determine differences between groups and taxon boundaries when many characters are studied. According to the latest version of the Biological Species Concept, subspecies are defined on the basis of geographical distribution, concordant characters, and unique diagnosable properties. However, there is no objective criteria that defines the boundaries of the subspecies category. It is questionable that the subspecies concept is operational for the Phylogenetic Species Concept.
BİLGİN, C. CAN (1999) "The Use of Coefficient of Difference (CD) in Systematic Zoology and the Subspecies Concept," Turkish Journal of Zoology: Vol. 23: No. 7, Article 2. Available at: https://journals.tubitak.gov.tr/zoology/vol23/iss7/2