Examine Markers of Potential Selection Bias in Relation to Measures of Association

Based on the postulated mechanism for the occurrence of selection bias, predictions can be made regarding segments of the study base in which the problem would be more or less substantial, even if it pervades all groups to some degree. Table 5.4. Median Serum Carotenoid Concentrations (10th-90th Percentile) for African-American Cases and Controls, and White Cases and Controls, from a U.S. Multicenter Case-Control Study of Prostate Cancer, 1986-1989 Table 5.4. Median Serum Carotenoid...

Evaluate Known Consequences of Exposure

In some instances, exposures are known to have specific health consequences, and verification of the expected associations with those other outcomes offers a means of assessing the accuracy of exposure assessment. Presumably the principal research question concerns potential health consequences of exposure that are not fully understood or the study would be of little value. When data can be obtained to assess whether strongly expected associations with exposure are found, in the same sense as a...

Apply Knowledge of Confounding Based on Other Studies

Although a specific confounding variable may not have been measured or not measured well in a particular study, there may be other similar research that can help to assess the potential for confounding. Previous research may provide a basis for judging whether the required confounder-exposure and confounder-disease associations are likely to be present, and even suggest the direction and magnitude of confounding that is likely to result from those associations. Alternatively, previous studies...

Subject Selection in Case Control and Cohort Studies

A case-control study involves the comparison of groups, namely cases and controls, in order to estimate the association between exposure and disease. By choosing persons with the adverse health outcome of interest (cases) and a properly constituted sample from the source population (controls), we assess exposure prevalence in the two groups in order to estimate of the association between exposure and disease. A cohort study also has the same goal, estimating the asso ciation between exposure...

Integrated Assessment Of Potential For Selection Bias In Casecontrol Studies

The initial step in evaluation of selection bias is to articulate in very specific terms what is required for the cases to be enrolled as cases in the study. The eligibility criteria start with the structural requirements based on age, gender, and geography, but often go well beyond these attributes to include medical care determinants, behaviors, and even attitudes. With the cases well defined, the study base will also be conceptually well defined as the population experience that generated...

Consider Discretionary Health Care of Cases and Controls

In addition to experiencing the biological process of disease occurrence, in many studies the identified cases of disease either recognized a manifestation of the biological problem and sought treatment for it, or had incidental contact with the health care system that led to a diagnosis. In some instances, the investigators engage in systematic case-finding and eliminate the discretionary component of being diagnosed, but often the cases in case-control studies are identified through a health...

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...

Create Subgroups with Accurate Ascertainment or Non Differential Underascertainment

Estrogens Endometrial Cancer Hulka

For a number of reasons, accurate ascertainment of disease may be unattainable for the entire study population. There may be subgroups within the study population, however, in which disease ascertainment is accurate. Some diseases are largely asymptomatic, for example carcinoma in situ of the cervix, but among women who obtain annual Pap smear screening the diagnosis is likely to be virtually complete. Valid results can therefore be obtained within the subset of the population in which...

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...