Inferences about the variability of means from censored data.

In recent years, intensive biological monitoring studies have been carried out on the Niagara River by the Ontario Ministry of the Environment. The basic objective was to determine the relative bioavailability of trace contaminants at various locations in the river, and to identify sources. A recurring difficulty encountered with the generated data is that substantial portions of sample concentrations of many toxic pollutants are below the limits of detection established by analytical laboratories. Under the assumption that the distribution of the data is log normal, the likelihood ratio test for testing the equality of several means for type I censored data is derived and its use for evaluating the spatial variability of trace contaminants in the river is illustrated.