Unless data are observed within a very specific experimental design, ecological data are often correlated. As an example, consider the problem of estimating stream sulfate concentrations in the eastern United States. We consider data collected as part of the EPA?s Environmental Monitoring and Assessment Program (EMAP). The sample sites were mainly located in Pennsylvania, West Virginia, Maryland, and Virginia. For more details about this example and the issues described below, see Irvine et al. (2007). The response Y � [Y1, . . . , Yn]0 is stream sulfate concentration at each of the n stream sites. In this simplified example we consider four predictors [X1, . . . , X4] which are geographic information system (GIS) derived covariates [...]
