Van Sickle et al. 2006

Van Sickle, J., J. L. Stoddard, S. P. Paulsen, and A. R. Olsen. 2006. Using Relative Risk to Compare the Effects of Aquatic Stressors at a Regional Scale. Environmental Management 38:1020-1030.

The regional-scale importance of an aquatic stressor depends both on its regional extent and on the severity of its effects in ecosystems where it is found. Sample surveys, such as those developed by the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program (EMAP), are designed to estimate and compare the extents, throughout a large region, of elevated conditions for various aquatic stressors. In this paper, we propose using relative risk to assess, in a complimentary way, the severity of each stressor's effect on a response variable that characterizes aquatic ecological condition. Specifically, relative risk measures the strength of association between stressors and response ecological conditions that can be classified as either "good" (i.e., reference) or "poor" (i.e., different from reference).  We present formulae for estimating relative risk and its confidence interval, adapted for the unequal sample inclusion probabilities employed in EMAP surveys. For a recent EMAP survey of streams in 5 Mid-Atlantic states, we estimated the relative extents of 8 stressors as well as their relative risks to aquatic macroinvertebrate assemblages, with assemblage condition measured by an index of biotic integrity (IBI). For example, an excess sedimentation stressor had a relative risk of 1.60 for macroinvertebrate IBI, with the meaning that poor IBI conditions were 1.6 times more likely to be found in streams having poor conditions of sedimentation than in streams having good sedimentation conditions. We show how stressor extent and relative risk estimates, viewed together, offer a compact and comprehensive assessment of the relative importances of multiple stressors.