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Table 1.Count of AP-42 compounds at each rating level(A is strongest;total of 43 compounds). Count Rating WIAC AP-42 A 12 4 B 14 21 C 2 8 D 6 6 E 9 4 The overall rating of the WIAC database is essentially the same as that for U.S.EPA's.For example when the letter grade is expressed as a numeric value(e.g.,A= 1,B=2,etc.),the average ratings for the WIAC and.U.S.EPA data sets are identical. Nondetects AP-42 directs that in general nondetect values should be halved then treated as"real"data. However if a nondetect exceeds by two times the maximum of the detects for a compound,then it should be discarded. It appears that the AP-42 guidance directs that this should be done on-a facility-by-facility basis as well as on an emission category basis.However the,guidance is unclear.A conservative approach was taken by eliminating only nondetects that were more than double the maximum detection among all facilities. AP-42 also directs that if all values are nondetects then the result should be clearly indicated as such.U.S.EPA does not indicate which values reported within the LFG portion of AP-42 are nondetects. Data Averaging AP-42 specifies that data from a single landfill are to be arithmetically averaged.The result from each landfill is then further averaged using an arithmetic average,geometric mean,or median depending on whether the landfill data are normally distributed,lognormally distributed,or neither,respectively.The distribution type was determined for each compound using the probability plot correlation coefficient method.5 Where fewer than four landfills reported a compound,the distribution type could not be determined.Instead,the distribution type originally used by U.S.EPA in AP-42 was employed.The distribution type was found to differ from U.S. EPA's for sixteen compounds. The WIAC data set was averaged using both U.S.EPA's original and the newer WIAC's distribution types(see Table 2).The original distribution types were applied so that an"apples to apples"comparison was possible.Doing otherwise could either create or obscure differences between the data sets.The averages calculated based on U.S.EPA's and WIAC's averaging types are shown in the WIAC column labeled"1"and"2",respectively.Values in WIAC column 2 having a different distribution type are highlighted in gray.The results using the two data averaging methods are discussed in Data Summary below. Codisposa! Landfills Because of detected statistical differences,EPA developed separate codisposal and municipal solid waste(MSW)only default AP42 levels for toluene and benzene.All other default values 5 This test was developed by J.J.Filliben in 1975 as reported in"Statistical Training Course for Ground- Water Monitoring Data Analysis",sponsored by the U.S.Environmental Protection Agency Office of Solid Waste, 1992. 3