Dr. Don Hendricks -3- August 28, 1972 and express this as a proportion: Standard error as proportion = ——- = ——= = so that approximate confidence limits can be expressed as (for accuracy one might use t-variate as multiplier for the smaller sample sizes, but the present procedure seems adequate for making judgements; also we will later bring)the effect of skewness on confidence limits): Confidence limits as proportion of mean = +2 SE(prop.) = + 2 An Thus if we know or assume the value of the coefficient of variation appropriate for the Atoll, one can calculate the sample sizes necessary for statements such as the following: "The probability that the true mean concentration of plutonium in soil samples from Japtan Island wil] be within + 5% of the observed mean determined from n samples is .95". In other words, we can specify a percentage interval within which we can have high confidence that the actual concentration will fall, given an advance estimate of C (the coefficient of variation). Note that all we need is C, since the actual concentrations do not enter the calculations -- we make the statements as proportions or percentages. Some actual estimates of C for soil plutonium are as follows: Samples Locations Size Bikini (page 12 of NVO-97/ Summary report Coefficient of variation 1 74 12 -42 of the 1969 and 1970 Bikini Surveys) Eniwetok (Data on samples from Janet, by phone from 0. Lynch) Eniwetok (9 composites of 3 each from Phase IIA 9 - -75 (1.30)* islands; Alice to Edna) Rocky Flats (HASL-235) Nevada Test Site (GMX study) *adjusting for composites 33° ~ Stratum I 3.6 10 .66 Stratum I] 4 55 - Stratum IV 6 1.36