700 quality control but a bad standard because the standard deviations of the two middle groups are not significantly different statistically. We have not had the opportunity to investigate this type of possible cause and by now, since the data are 2 years old, tions used on these data may not be the standards and calibra- recoverable. Had the possibility of 600 this situation been suspected in advance of the actual cross-check test, sufficient information could have been collected to investigate this and other possible causes. It is obvious that if any reasonable quality control standard were applied to the total data set of this example, represented by the broken Estimated Population of Sampled Data Figure 3, 400 300 DATA VALUES 500 line curve in Figure 3, the result would be an unreasonable set of acceptance limits as wall as a loss of information contained by the data. Perhaps our current uneasiness with acceptance rules is due to our intuitive recognition of this unreasonableness. We err in placing the blame on the acceptance rule rather than on the type of data we work with. CONCLUSION This is just one of about 50 quality control data sets involving 10 different radioisotopes that have so far been examined in ovr laboratory, all but one showed a statistically significant (maximum likelihood ratio test) mixture of distributions. Similar statistical work has also found such mixtures in thermoluminescent dosimeter data, plutonium in soil data, and air pollutant data. The plutonium in soil mixture-of-distributions has been identified as caused by two distinctive sizes of plutonium particles by using autoradiographs of soil samples; about 85 percent of the plutonium in the soils found at the Nevada Test Site is in the form of an extremely fine dust and 15 percent is particles large enough to be counted under a dissecting microscope. This has been called the "hot particle problem.” Mixtures of distributions are a well known and accepted phenomena in medical statistics where the components distributions are typically identified as sick and well patients, male and female, and so on. Our quality control data differ from the hot particle problem and the medical examples cited above in the reasons for the structure in the data. The structures in the hot particle situation and in the medical examples are due to a true structure in the media sampled. In our quality control data, aliquots of a single homogeneous laboratory- prepared medium are sent to the participants, and the structure is introduced during the sample analysis procedure. There is nothing in the present statistical analysis to @istinguish the various causes of a heterogeneous data structure. whatever the cause. The 599 HOH 40 JAUND IWWHON 40 31¥NIGHO 598 statistical analysis is the same The cause can only be defined by follow-up investigations.