Opinion ID: 519994
Heading Depth: 3
Heading Rank: 1

Heading: The EPA's Use of Weighted Averaging in Deriving the

Text: Long-Term Averages 257 To derive the BAT limitations the EPA averaged over a long term the amount of the toxic discharged by what it considered to be the best plant or plants in the data base using BAT technology. It then multiplied this long-term average by a variability factor, a number always greater than one, to account for its estimate of the reasonable variation from the average that could be expected by the several best-performing plants in the data base. The product was the BAT limitation for that toxic. 258 A toxic may be present in a discharge in such a small amount that it cannot be detected by state-of-the-art analytical methods, that is, the amount of a toxic may be less than the analytical minimum. Generally, the analytical minimum for the detection of a toxic is 10 parts per billion (ppb), a proportion that, following an analogy employed by CMA at oral argument, is comparable to one minute in two thousand years. For the purpose of calculating long-term averages the EPA assumed that the amount of an undetected substance contained in a discharge (the non-detect value) equalled the analytical minimum; in other words, if the toxic might be present, but only in such a small concentration that it could not be detected by currently-used analytical methods, the EPA assumed the toxic was present at the analytical minimum, generally 10 ppb. None of the petitioners objects to the EPA's use of non-detect values in the calculation of long-term averages, presumably because assigning the analytical minimum to a reading in which a specific toxic could not be detected had the effect of raising the long-term average and thus raising the effluent limits. CMA objected to weighted averaging of such values, however, because this lowered the limitations. 259 The long-term averages were calculated by averaging detectable values and non-detect values. The EPA calculated the average of the assigned non-detect values and the average of detectable values for the data base and then weighted the averages according to the proportions of all non-detect and detectable values reported for plants in the data base for a given pollutant. The long-term average for a particular plant was calculated as the weighted sum of the averages of detect and non-detect values reported for plants in the data base. 198 260 CMA contends that the EPA's weighted averaging of non-detect values significantly lowered the long-term averages it calculated from measured effluent concentrations, and thus lowered the OCPSF effluent limits for at least 27 pollutants. CMA concludes that this was without justification or support, hence arbitrary. 261 The choice of statistical methods is committed to the sound discretion of the Administrator. 199 Weighted averaging is a recognized statistical method for adjusting a data set when the data for particular units, plants in this case, are not represented by a comparable number of readings or samples. When some units in a data set are represented by fewer readings than other units in the set, the units represented by fewer readings should be given less weight in the average, i.e., discounted through weighted averaging. Conversely, those units in a data set based on a larger number of readings should be given greater weight in the average. Including a unit for which there is a smaller number of readings in a calculation, without discounting it by weighted averaging, would distort the average. 200 262 CMA correctly contends that use of a different averaging method would yield different results. Indeed, several different methods might have been used, each producing a different result. As the First Circuit has noted, however, [t]he choice of any given method may mean that an alternative method would yield different results. The necessary corollary ... is that any other system chosen would be open to the same criticism. We will not leave the Agency so vulnerable. 201 We agree, provided, of course, that there is no demonstration that the Agency's method was chosen arbitrarily or merely for the purpose of achieving a predetermined prejudicial effect. The EPA's use of weighted averaging to determine the long-term averages was not an abuse of discretion.