In the face of complexity, ecologists often strive to identify models that capture the essence of a system, explaining the observed distribution and perhaps ultimately permitting prediction. A first step toward this aim is to collect data on the response of interest, together with data on factors that it is believed might influence that response. Frequently data are observational (i.e. the variance in the data set has not been generated by experimental manipulation) leading to difficulties in determining which causal factor or factors best explain the observed responses. In these situations, scientific possibility is limited to describing the system and identifying models consistent with the observed phenomenon. One of the most commonly used techniques for this purpose is multiple regression or, more generally, a general linear model with multiple predictors. The statistical theory underlying this methodology is well understood