In the population of Utirik (158 persons, all ages) followed for 21 years, the accumulated person-years of observation are estimated at 405708 3140. In such a group, expection of thyroid cancer would be as follows: (1) Based on an average for all registries listed in "Cancer Incidence on Five Continents" (2.5/100,000 per year): .08 cases, upper 95% confidence limit .66 (2} Based on incidence rates in the Marshal Islands, ‘ a lagen meW-/429 \} 4, (dates), excluding cases in exposed individuals (2.0/100,000 per year): .06 cases, upper 95% confidence limit .54 (3) Based on estimates of the risk per rad thyroid dose per year observed in Rongelap and Ailingnae combined (3.9/1,000,000/ : Oo | - | rad/year, person~yoarseat~riskrest-3140, thyroid dose 50 rads j= 157,000 person-rad-years) : -61 cases, upper 95% confidence limit 2.17 ~~ J : A c Conclusions: 0 , 4 3 q 1), (1) ~ 5 unlikely to be due to chance at conventional levels of statis- UL 3 a tical significance - if the rates do not exceed normal ; Ue population rates. The occurrence of 2 cases by chance is extremely C Qi= ~ unlikely. ) iy 3% ¢ <3 & iz o E 8 The occurrence of even 1 case of thyroid cancer in Utirik is , . , ; oo po Zz = - a (2) an os The observed number of thyroid cancers is higher than would be , : expected on the basis of experience in the exposed populations . . teat $ . oo . 7 . . ‘ of Rand A, but does not exceed the upper 95% confidence limit i 39 § 28 8 - of the expected value. : Caveats (1) In computing expected values, no account has been taken of age and sex differences between populations. The incorporation of i | Vorique adjustments for these factors is unlikely to change the above conclusions.