The concentration-coverage curve: a tool for ecological studies.

BACKGROUND: A selective approach may be used in an ecological study where the aim is to choose a subset of units of analysis (UAs) and produce interpretations about a population of interest (PI) based solely on those UAs. The results for the PI will be reliable if that population is concentrated in the selected UAs and rare in other UAs. This article presents a graphical tool that helps determine whether these conditions are satisfied. DATA AND METHODS: Data on the Inuit and Metis ancestry populations from the 1996 Census of Canada are used for illustrative purposes. Based on a classification statistics table, a concentration-coverage curve can be created for a given PI. The shape of the curve indicates whether it is possible to choose a threshold that will yield both adequate concentration and adequate coverage of the PI. RESULTS: The concentration-coverage curve shows that, among Aboriginal peoples living in rural areas, the Inuit population is classifiable, but the Metis population is not. INTERPRETATION: This method can be applied to any ecological study focussing on the proportion of individuals sharing a single characteristic defined by a binary variable.