Inferences From Epidemiologic Evidence Efficacy Of Breast Cancer Screening

A specific example illustrates the range of potential questions that can be applied to epidemiologic data (Table 2.1) and how a given study may answer some questions effectively and others rather poorly. Assume that the exposure under study is participation in a regular mammography screening program and the disease of interest is fatal breast cancer. Such a study has potential relevance to many questions. 1. What is the mortality rate among women who participated in the mam-mography screening...

Assessing Whether Misclassification Is Differential By Exposure

In evaluating the influence of possible disease misclassification, careful consideration must be given to whether the errors are likely to differ as a function of the exposure of interest. Misclassification of disease that is differential by exposure directly distorts the measure of effect, and therefore even relatively modest differences are capable of generating spurious increases or decreases in the estimated measures of effect. Non-differential misclassification of disease may produce no...

Definition And Theoretical Background

Confounding is one of the fundamental methodological concerns in epidemiology. Although rarely as explicitly examined in other disciplines, and sometimes identified with different terminology, confounding is also a theme in other branches of science. Wherever one is concerned with identifying causal associations, whether through observational or experimental studies, a key focus of study design and analysis is to address the potential effects of confounding. The concept of confounding can be...

Example Epidemiologic Research On The Relation Between Dichlorodiphenyltrichloroethane Ddt Exposure And Breast Cancer

To illustrate the strategy, if not the complete implementation, of an evaluation of sources of error in epidemiologic studies, the first major epidemiologic study on persistent organochlorides and breast cancer by Wolff and colleagues 1993 is examined. The hypothesis they considered was that persistent organochloride compounds, including the pesticide DDT, its metabolite dichlorodiphenyldi-chloroethane DDE , and the industrial pollutant, polychlorinated biphenyls PCBs , might increase the risk...

Identification of Subgroups with Nondifferential Exposure Misclassification

Because the effects of errors in exposure that are the same for diseased and non-diseased individuals tend to produce more predictable errors, often leading to bias toward the null value, there is value in trying to create strata in which the error is likely to be nondifferential. It would be preferable, of course, to avoid error altogether, but stratification to achieve nondifferential misclassification is still of benefit. When a determinant of exposure accuracy, for example, educational...

Selection Bias And Confounding

There are two closely related processes that introduce bias into the comparison of exposed and unexposed subjects in cohort studies. When there is a distortion due to the natural distribution of exposures in the population, the mixing of effects is referred to as confounding. When there is a distortion because of the way in which our study groups were constituted, it is referred to as selection bias. In our hypothetical cohort study of dietary fat intake and prostate cancer, we may find that...

Compare Unexposed Disease Rates to External Populations

Comparison of the absolute rate of disease occurrence in the unexposed portion of the cohort with the rate of disease in an appropriate external reference population may help to determine whether the unexposed group is likely to provide a suitable benchmark of comparison for the exposed study group. The purpose of this strategy is to evaluate whether the unexposed group is likely to be effective in its role of measuring what the disease rate would have been in the exposed group had they not...

Inferences From Epidemiologic Evidence Alcohol And Spontaneous Abortion

Another illustration of the different levels of inference about epidemiologic evidence and the challenges at each level concerns the relation between maternal alcohol intake in early pregnancy and the risk of spontaneous abortion, pregnancy loss prior to 20 weeks' gestation. The initial descriptive goal is to accurately measure alcohol consumption in a population of pregnant women, and then to monitor the incidence of spontaneous abortion. These measurement issues present a substantial...

Assess Consequences of Inaccurate Confounder Measurement

The markers of confounding that are available inevitably fall short, to varying degrees, of the ideal, just as exposure measures generally do not capture precisely the construct that they were intended to measure. In seeking to measure and control for confounding factors, we look for handles on the basis for non-comparability of the unexposed and exposed groups, a challenging mission that is almost guaranteed to be less than totally successful. The groups to be compared are not naturally...

Temporally Relevant Exposure

The timing of exposure relative to disease occurrence is among the most underappreciated aspects of exposure assessment and thus merits special emphasis. Some exposures are constant over time, such as genetic constitution or attributes defined at birth. However, exogenous exposures such as diet, medication use, social circumstances, and chemical pollutants vary over time, often substantially. Any choice of exposure measure implicitly or explicitly includes an assumption about the exposure...

Selection of Controls from the Study Base

A key concept in case-control studies that guides control selection is the study base Miettinen, 1985 , defined simply as the person-time experience that gives rise to the cases. Conceptually, the study base is populated by the people at risk of becoming identified as cases in the study if they got the disease during the time period in which cases are identified. Note that this is more than just being at risk of developing the disease in that membership in the study base also requires that if...