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For example, a study of the effect of smoking on heart disease will suffer selection bias if subjects of the study are volunteers and the decision to volunteer is affected by both being a smoker and having a family history of heart disease. The association will be biased upward because of the additional disease among the exposed smokers caused by genetics. Hospital-based studies, which are relatively common among researchers located in medical centers, illustrate the problem.
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Suppose an association is found between coffee drinking and coronary heart disease in a study using hospital patients as controls. The problem is that the hospitalized control group may include individuals who had been advised against drinking coffee for medical reasons, such as to prevent the aggravation of a peptic ulcer. In other words, the controls may become eligible for the study because of their medical condition, which is in turn related to their exposure status—their likelihood of avoiding coffee.
If this is true, the amount of coffee drinking in the control group would understate the extent of coffee drinking expected in people who do not have the disease, and thus bias upwardly i. Just as cases and controls in case-control studies should be selected independently of their exposure status, so the exposed and unexposed participants in cohort studies should be selected independently of their disease risk. A further source of selection bias occurs when those selected to participate decline to participate or drop out before the study is completed.
Many studies have shown that individuals who participate in studies differ significantly from those who do not. If a significant portion of either study group declines to participate, the researcher should investigate whether those who declined are different from those who agreed. The researcher can compare relevant characteristics of those who.
The case-control method : design and applications
William B. Hershel Jick et al. When unexposed controls may differ from the exposed cohort because exposure is associated with other risk or protective factors , investigators can attempt to measure and adjust for those differences, as explained in Section IV. See also Martha J. Similarly, if a significant number of subjects drop out of a study before completion, the remaining subjects may not be representative of the original study populations. The researcher should examine whether that is the case. The fact that a study may suffer from selection bias does not necessarily invalidate its results.
A number of factors may suggest that a bias, if present, had only limited effect. If the association is particularly strong, for example, bias is less likely to account for all of it.
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In addition, a consistent association across different control groups suggests that possible biases applicable to a particular control group are not invalidating. Similarly, a dose—response relationship see Section V. C , infra found among multiple groups exposed to different doses of the agent would provide additional evidence that biases applicable to the exposed group are not a major problem. Information bias is a result of inaccurate information about either the disease or the exposure status of the study participants or a result of confounding.
In a case-control study, potential information bias is an important consideration because the researcher depends on information from the past to determine exposure and disease and their temporal relationship. Research has shown that individuals with disease cases tend to recall past exposures more readily than individuals with no disease controls ; this creates a potential for bias called recall bias.
For example, consider a case-control study conducted to examine the cause of congenital malformations. Information bias can be a problem in cohort studies as well. When exposure is determined retrospectively, there can be a variety of impediments to obtaining accurate information. Similarly, when disease status is determined retrospectively, bias is a concern. The determination that asbestos is a cause of mesothelioma was hampered by inaccurate death certificates that identified lung cancer rather than mesothelioma, a rare form of cancer, as the cause of death.
See I. Steven S. Clinical Epidemiology 87 See Brock v.
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We note that the court was mistaken in its assertion that a confidence interval could correct for recall bias, or for any bias for that matter. Confidence intervals are a statistical device for analyzing error that may result from random sampling. Systematic errors bias in the design or data collection are not addressed by statistical methods, such as confidence intervals or statistical significance. See Green, supra note 47, at —68; Vincent M. Brannigan et al. Mothers of children with malformations may recall an inconsequential fever or runny nose during pregnancy that readily would be forgotten by a mother who had a normal infant.
Even if in reality the infection rate in mothers of malformed children is no different from the rate in mothers of normal children, the result in this study would be an apparently higher rate of infection in the mothers of the children with the malformations solely on the basis of recall differences between the two groups.senjouin-kikishiro.com/images/dutapevi/1112.php
Environmental sanitation conditions and health impact: a case-control study
Bias may also result from reliance on interviews with surrogates who are individuals other than the study subjects. This is often necessary when, for example, a subject in a case-control study has died of the disease under investigation or may be too ill to be interviewed. There are many sources of information bias that affect the measure of exposure, including its intensity and duration. Exposure to the agent can be measured directly or indirectly.
Biological markers, however, are only available for a small number of toxins and usually only reveal whether a person was exposed.
Thus, in Newman v. See In re Paoli R. See Gary E. There are different definitions of dose, but dose often refers to the intensity or magnitude of exposure multiplied by the time exposed. See Sparks v. Owens-Illinois, Inc. Monitoring devices also can be used to measure exposure directly but often are not available for exposures that have occurred in the past. For past exposures, epidemiologists often use indirect measures of exposure, such as interviewing workers and reviewing employment records.
Thus, all those employed to install asbestos insulation may be treated as having been exposed to asbestos during the period that they were employed. However, there may be a wide variation of exposure within any job, and these measures may have limited applicability to a given individual. Thus, retrospective studies, which are often used for occupational or environmental investigations, entail measurements of exposure that are usually less accurate than prospective studies or followup studies, including ones in which a drug or medical intervention is the independent variable being measured.
Other definitions of dose may be more appropriate in light of the biological mechanism of the disease. For a discussion of the difficulties of determining dose from atomic fallout, see Allen v. The timing of exposure may also be critical, especially if the disease of interest is a birth defect. In Smith v. Ortho Pharmaceutical Corp. The researchers had defined exposure as receipt of a prescription for spermicide within days of delivery, but this definition of exposure is too broad because environmental agents are likely to cause birth defects only during a narrow band of time.
A different, but related, problem often arises in court. Lamb, 46 P. Gislaved Gummi AB, F. Bayer Corp. Pittsburgh Corning Corp. V-J Auto Parts, Inc. Frequently, occupational epidemiologists employ study designs that consider all agents to which those who work in a particular occupation are exposed because they are trying to determine the hazards associated with that occupation. Isolating one of the agents for examination would be difficult if not impossible.
These studies, then, present difficulties when employed in court in support of a claim by a plaintiff who was exposed to only one or fewer than all of the agents present at the worksite that was the subject of the study. Kirby Inland Marine Inc.
The route e. Even with environmental monitoring, the dose measured in the environment generally is not the same as the dose that reaches internal target organs.
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If the researcher has calculated the internal dose of exposure, the scientific basis for this calculation should be examined for soundness. In assessing whether the data may reflect inaccurate information, one must assess whether the data were collected from objective and reliable sources. Medical records, government documents, employment records, death certificates, and interviews are examples of data sources that are used by epidemiologists to measure both exposure and disease status.
If different data sources are used to collect information about a study group, differences in the accuracy of those sources may affect the validity of the findings. For example, using employment records to gather information about exposure to narcotics probably would lead to inaccurate results, because employees tend to keep such information private. If the researcher uses an unreliable source of data, the study may not be useful. The kinds of quality control procedures used may affect the accuracy of the data.
For data collected by interview, quality control procedures should probe the reliability of the individual and whether the information is verified by other sources. For data collected and analyzed in the laboratory, quality control procedures should probe the validity and reliability of the laboratory test.
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Information bias may also result from inaccurate measurement of disease status. The quality and sophistication of the diagnostic methods used to detect a disease should be assessed. If, for example, many of the subjects refused to be tested, the fact that the test used was of high quality would be of relatively little value. See also Bernard D. Even these sources may produce unanticipated error.
Identifying the causal connection between asbestos and mesothelioma, a rare form of cancer, was complicated and delayed because doctors who were unfamiliar with mesothelioma erroneously identified other causes of death in death certificates. See David E. Health Rep. Novartis Consumer Health, Inc. The experts failed to examine whether there was misclassification in the controls, which, if it existed, would tend to incorrectly diminish any association.
The scientific validity of the research findings is influenced by the reliability of the diagnosis of disease or health status under study.