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, > i In the population of Uti rik (158 persons, all ages) followed for In the population of Uti rik (158 persons, all ages) followed for 70616621 years,the accumulated person-years o f observation are estimated a t 2J40. In such a group, expection of thyroid cancer would be as follows: (1) Rased on an average for all registries listed in "Cancer Incidence on Five Continents" (2.5/100,000 p e r year): .OS cases, upper 95% confidence limit .66 (2) D:tsed on incidence rates inthe Marshal Islands,(dates), i excluding cases in exposed individuals (2.0/100,000 per year) : (3) Based on estimates of the risk per rad thyroid dose per year observed i n Rongelap and Ailingnae combined (3.9/1,000,000/ .61 cases, upper 95% confidence linit 2.17 (1) The occurrence o f even 1 case of thyroid cancer in Uti rik is unlike lk to be due to chance ' a t conventional levels of statist ical significance - if the rates do not exceed normal population rates. The occurrence of 2 cases by chance i s extremely The observed number of thyroid cancers i s higher than would be expected on the basis of experience inthe exposed populations ofR and A,but does not exceed the upper 95% confidence limit of the eq ec ted value. (1) In computjng expected values, no account has been taken of age and sex differ cn ccs between populations. The incorporation of adjustments for thc se factors is unlikely to change the above
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(2) The above calculations have ignored the possibility that some unknown proportion o f person-years of observation are irrelevant because of the latent period between exposure and detection of cancer. Subtraction of these person year9 would increase the difference between observed and expected values derived from non-exposed populations. It should be borne i n mind that the Marshallese have been more carefully observed for thyroid disease than the general populations from which expected values have been derived. It is curious that the Uti rik population evidences no increase i n benign thyroid lesions relative to the unexposed population of Rongelap.