for cancer that is used here. One reason for this definition is the lack of epidemiologically based research using morbidity indicators. The advantages and disadvantages of this measure are discussed further in the section entitled ''Preventability of Burden."

In this chapter, an ecologic study indicates the examination of population-based rates. Case-control studies involve rates of prior screening in cases (e.g., persons who have died from the cancer) as compared with such rates in controls (e.g., persons who have not died from the cancer and usually have not contracted it). Nested case-control studies refer to case-control studies conducted within a longitudinally defined cohort in which careful attempts are made to classify all members as to level of screening. This methodology limits the case-control comparisons to a fraction of the total cohort—for example, all cases who died from the cancer and five times as many age-stratified controls (the remaining controls are excluded).

Levels of assurance are based on whether evidence for efficacy involves the following: (1) different research designs, which may include ecologic and case-control studies, observational cohort studies, controlled nonrandomized studies, and randomized controlled trials; (2) the magnitude of the observed protective effect (e.g., an observed reduction in risk of 50 percent versus 10 percent); (3) the extent of care given to collecting data on the screening modality and on the outcome (e.g., death from the cancer), and the extent to which covariables such as socioeconomic status and race were considered; (4) the consistency in the results across different studies; (5) supporting data on sensitivity and specificity; and (6) the extent to which older age groups were studied.

BURDEN

Incidence, Mortality, Survival, and Prevalence

Age is the most consistent and strongest predictor of risk for cancer and for death from cancer. (The effect of age is so dramatic that incidence and mortality rates increase exponentially with age.)21,71Table 9-1 provides population-based incidence rates and incidence/mortality ratios from the Surveillance, Epidemiology, and End Results (SEER) program in the United States. The adult epithelial cancers are more common in persons over the age of 50 than in those under that age, and the rates of incidence and mortality increase with each decade of life after age 50. The incidence/mortality ratios imply that death rates for cancer generally increase with age more rapidly than incidence rates.



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