Dr. Don Hendricks -4- August 28, 1972 We have arbitrarily selected C=.7 as the value to use here. Other evidence suggests that this will not be a bad guide for other radioisotopes, and that the results should apply reasonably well to those islands contaminated mainly by fallout. If the sampling is done along a very distinct gradient in concentrations, then the above guides don't hold, as for example as shown in the Rocky Flats data. If one has some evidence on whict to sketch in several levels of contamination (as was available at the GMX site on NTS) then the variability within such a subdivision should again be roughly as indicated. The consequences for the Eniwetok study seem to be that we can feel reasonably secure in predicting confidence limits (as %) for situations other than on those islands where ground zeros existed, where rather larger samples are indicated for comparable confidence limits. The above calculations can be summarized in the following table: n (sample size) Approximate confidence limits as % of mean concentration 10 + 44% 20 + 31% 30 + 26% . 40 50 + 22% “ + 20% 60 + 18% 100 + 14% 200 + 10% These results are simply calculated as 1.4/59 so one can easily find values for other sampleé sizes, or can increase the numerator (which is 2(.7) or twice the coefficient of variation) to some larger value to reflect supposedly greater variation. From this evidence, we don't recommend that sample sizes as smal? as 10 be regarded as anything but rough guides to relative concentrations. Samples of 30 or more may be sufficient to make some reasonably satisfactory. estimates. However, if one wants to be fairly precise, even 100 samples is not too many, since we still have a fdarly wide range (+14%) for the true mean. Perhaps it is worth repeating here that these confidence