Selection of threshold values in geochemical data using probability graphs

A method of choosing threshold values between anomalous and background geochemical data, based on partitioning a cumulative probability plot of the data is described. The procedure is somewhat arbitrary but provides a fundamental grouping of data values. Several practical examples of real data sets that range in complexity from a single population to four populations are discussed in detail to illustrate the procedure.The method is not restricted to the choice of thresholds between anomalous and background populations but is much more general in nature. It can be applied to any polymodal distribution containing adequate values and populations with appropriate density distribution. As a rule such distributions for geochemical data closely approach a lognormal model. Two examples of the more general application of the method are described.