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An ability-based guessing model is formulated and applied to several data sets regarding educational tests in language and in mathematics. The formulation of the model is such that the probability of a correct guess does not only depend on the item but also on the ability of the individual, weighted with a general discrimination parameter. By so doing, the possibility that the individual uses his or her ability to some extent for differentiating among responses while guessing is also considered. Some important properties of the model are described and compared with analogous properties of related models. After simulation studies, the model is applied to different data sets of the Chilean Sistema de Medición de la Calidad ń (SIMCE) tests of mathematics and language. The conclusion of this analysis seems relevant—namely, that the examinees use their ability to guess in the language test but not in the mathematics test.

We report an autopsy study of gallbladder cancer prevalence in Chile, where the risk of this disease is among the highest reported world-wide. In 14,768 autopsy protocols obtained from 3 university hospitals, 45% of women and 20% of men older than 20 years had gallstone disease (the major known risk factor for gallbladder cancer). The prevalence of gallbladder cancer in Chileans was compared to that found in a Swedish-Czechoslovakian autopsy study previously published. These countries were chosen because of their high frequency of gallstone disease. The comparison was performed by using logistic regression models adjusting for possible differences in the age-sex structure or the true incidence of gallstones in both populations. We found that the most important single risk factor for gallbladder cancer in Chile was gallstone disease, with an estimated effect on the logistic scale meaning that the cancer risk for subjects with gallstones is seven times higher than for those without the disease. Second in importance was the risk for sex, women being 2.8 times higher than for men. The estimated difference in the sex composition and the incidence of gallstones resulted in 17.9% higher odds of cancer in Chile than in Sweden and Czechoslovakia. However, this difference was not significant. This study suggests that the major etiologic factors of gallbladder cancer in Chileans, Swedes and Czechoslovakians are primarily related to gallstone disease.

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Increasing gall-bladder cancer mortality rate during the last decade in Chile, a high-risk areaGall-bladder cancer presents 01 large geographic variation in mortality rates (Diehl, 1980). The highest mortalities are found in Chile (Strom, 1984), Bolivia (Rios-Dalenz et al., 1983), North American Indians (Black et al., 1977), Mexicans (Alborez-Saavedra et al., 1980) and North-eastern Europeans (Strom, 1984). In contrast, gall-bladder cancer mortality rates are very low in Spain, in black Americans (Ditphl, 1980) and in Maoris from New Zealand (Fraumeni, 1975). Data from the Chilean National Institute of Statistics show a con.rtant increment of extra-hepatic biliary cancer mortality rates during the last 18 years (Serra et al., 1990). Identified risk factors for this cancer are gall-stones, age and sex (with a high female to male ratio) both in Chile (Nervi et al., 1988), and elsewhere (Strom, 1984). Large geographic variations in incidence strongly suggest the action of specific etiologic factors in the development of gall-bladder cancer. The principal risk factor is gall-stone disease in Chile and other countries. Potential environmental risk factors are primarily related to gall-stone formation (Bull and Nervi, 1987). Further evidence linking gall-stone disease and gall-bladder cancer has been the finding of a reciprocal relationship between cholecystectomy rates and mortality from gall-bladder cancer (Diehl and Beral, 1981).Cholesterol gall-stones are exrremely common in Chile, approximately 45% of women and 20% of men older than 20 years having gall-stone disease (Covarrubias et al., 1984). Cholecystectomy for gall-stone disease is the most frequent surgical procedure of the abdomen in Chile. It represents the second cause of hospitalization ajler obstetrical indications (Ministerio de Salud, 1988). More than 35% oj"cho1ecystectomies are performed because of acute cholecystitis in Chile (Paz et al., 1986).Mortality rates of extra-hepatic biliary tract cancer in Chile have consistently increased during the last decade from 7.841 100,000 in I980 to 9.96/100,00t3 in 1988, as shown in Table I . In contrast, mortality of hepatoma has remained essentially constant during the same period We tested the hypothesis that the increment in mortality rates from gall-bladder cancer may be the consequence of a reciprocal change in the number of cholecystectomies performed during the previous year(s). The assumption was made that increasing the number of gall-bladders at risk by decreasing the number of cholecystectomies carried out would increase the risk of gull-bladder cancer.As shown in Figure I , we found a striking, significant reciprocal correlation between the 2 variables. It is apparent from the data that the marked reduction of cholecystectomy rates between 1980 and 1988 in Chile may have increased the number of gall-bladders at risk for cancer. 'There is no evidence that the prevalence of gall-stone disease has changed in the last 20 years. Prevalence figures of autopsy material from 1966-1 970 (MarinoviC et al.,...