compared with expected effluent concentrations in receiving streams to predict the likelihood of in-stream toxicity. Despite recent strong challenges to the concept of NOEC and LOEC on statistical grounds (see, for example, Kooijman, 1996), NOECs and LOECs are widely used and are likely to remain so for regulatory purposes in the United States for years to come.

The key difference between effluent toxicity data and ambient toxicity data may be best conceptualized in terms of signal-to-noise ratio. Compared with most receiving waters, most effluents have a strong toxicity ''signal." On the other hand, the "noise level" for effluents tends to be lower than that of ambient waters. Thus, in general, the toxicity signal-to-noise ratio is higher for effluents than it is for ambient tests of receiving waters. This is important because it determines how the tests should be applied to maximize the information gained per dollar spent.

Figure 2

Statistical analysis flow path for reproduction data from Ceriodaphnia effluent toxicity tests (redrawn from Weber et al., 1989). NOTES: IC25 and IC50 refer to the concentrations of effluent or chemical that inhibits the measure of interest (such as growth or reproduction) by 25 or 50 percent, respectively. NOEC = no-observed-effect concentration. LOEC = lowest observed-effect concentration.