Therefore, any problem with nested case control studies must also be a problem for full cohort analysis. Selection bias arises either when cases in the study sample are not representative of cases arising from the source population study base or when controls are. Bias that might occur in cohort studies confounding bias selection bias information bias summary 3. Sources of bias in casecontrol studies recall bias occurs when the recall is better among cases than controls because of the presence of the disease. There are several types of selection bias, and most can be prevented before the results are delivered. Selection bias in case control studies may occur when. Bias and chance unaffected by sample size caused by the systematic differences in the casecontrol or exposedunexposed groups internal validity decreases as the sample size increases. Adjusting for selection bias in retrospective, case. The primary challenge in designing a case control study is the appropriate selection of cases and controls. Bias in full cohort and nested casecontrol studies. Methods to quantify selection bias are introduced together with analytical strategies to.
A third source of bias, confounding, is not considered in this article. Avoiding selection bias is a particular challenge in the design of casecontrol studies. Diagnoses case selection may be influenced by physicians knowledge of exposure example. When the probability of case identification among members of a primary base depends on a variable, the study base principle is violated and there can be selection bias, unless control selection depends proportionally on values of that variable. This can occur when exposure status influences selection. It discusses control selection, evaluation of selection bias in case control studies, and integrated assessment of potential for selection bias in case control studies. Several design options available in the planning stage of casecontrol studies are examined. Selection bias is a particular problem of casecontrol studies and is most likely to occur in situations where cases are derived from highly specialized clinical settings. Selection bias can present a serious problem for valid odds ratio estimation in case control studies as demonstrated. Generally, the only disadvantages to nested case control studies are the reduced precision and power due to sampling of controls, and the possibility of flaws in the sampling design or its implementation.
Assessing bias in case control studies proper selection of cases and controls kim suttontyrrell, drph case control studies are retrospective investigations in which a diseased group cases and a diseasefree group controls are compared with the aim of uncovering risk factors that differ between the groups. Jul 15, 2005 we discuss two classes of bias that arise in casecontrol studies, selection bias and information bias. Casecontrol study design is a type of observational study. Selection bias occurs if the recruited cases or controls are systematically different from the population of people they are intended to represent. An example of the role of selection bias is given in the worked example of casecontrol studies. It costs relatively less and can be conducted in a shorter time for a given disease, a casecontrol study can investigate multiple exposures when the real exposure is not known a casecontrol study is. Cases are often recruited using convenience samplingfor example, from hospital clinics, which are easily accessible. Department of epidemiology, university of pittsburgh, pa 15261. Selection bias in casecontrol studies selection bias is a particular problem inherent in casecontrol studies, where it gives rise to noncomparability between cases and controls. A casecontrol study is usually conducted before a cohort or an experimental study to identify the possible etiology of the disease. In a cohort study selection bias is when selectionparticipation of the exposed and unexposed subjects are related to disease status.
Selection biases pose a substantial threat to the validity of study findings for this design. Selection bias examples also occurs before subjects are identified for study diagnostic or workup bias. We describe examples of selection bias in casecontrol studies eg, inappropriate selection of controls and cohort. Selection bias can be introduced into casecontrol studies with low response or participation rates if the likelihood of responding or participating is related to both the exposure and the outcome. Selection bias controls are used to provide an estimate of the exposure rate in the population. We conclude that selection bias in casecontrol selection of subjects from rare disease registries occurs and that this can be overcome through casecontrol matching to minimize bias. Remember that in a case control study the controls are used to estimate the exposure distribution i. We call for better reporting and for evaluation of the potential for selection bias in all casecontrol studies, as well as, for the development of novel methods in control selection and recruitment. Selection bias can present a serious problem for valid odds ratio estimation in casecontrol studies as demonstrated. The primary challenge in designing a casecontrol study is the appropriate selection of cases and controls. Reducing selection bias in casecontrol studies from rare.
The authors have conducted a casecontrol study to evaluate the effect of various oha on bone mineral density bmd in patients with t2dm. Selection bias and its implications for casecontrol studies. In this poster we use directed acyclic graphs1 to model selection bias. Because casecontrol studies focus on women who have given birth to a baby with birth defects, casecontrol studies are usually far more statistically powerful than populationbased cohort studies of an equivalent size. This chapter focuses on selection bias in casecontrol studies. In a cohort study selection bias is when selection participation of the exposed and unexposed subjects are related to disease status. However, if present, such a bias would have wide implications for case control studies in general. These principles also apply to the design of cohort studies, as would be expected since the case control design is simply an efficient sampling technique to measure exposuredisease associations in a cohort or study base. Avoiding selection bias is a particular challenge in the design of case control studies. The effect of potential problems in obtaining comparable accuracy.
Selection bias in case control studies selection bias is a particular problem inherent in case control studies, where it gives rise to noncomparability between cases and controls. In contrast, in casecontrol studies participants are sampled at different rates. Case control study outcome is pulmonary disease, exposure is smoking. Casecontrol studies are typically prone to selection bias gis. Unbiased selection of cases and controls is very crucial to this research design. Remember that selection bias may occur in a cohort study if the rate of participa tion or the rate of loss to followup differ by both exposure and health outcome. Tools for evaluating the potential for selection bias in casecontrol studies include comparing measured exposure prevalence among controls to an external population and determining whether the exposure among controls follows expected patterns, examining exposuredisease associations in relation to markers of susceptibility to bias, adjusting. Casecontrol studies also are used for diseases that have long latent periods long durations between exposure and disease manifestation and are ideal when multiple potential risk factors are at play. Retrospective casecontrol studies are more susceptible to selection bias than other epidemiologic studies as by design they require that both cases and. Jan 30, 2015 casecontrol studies are typically prone to selection bias g is true. Topics covered include matching, controlcase ratio, choice of nested casecontrol or casecohort design, twostage sampling, and other methods that can be used for control selection. This work is licensed under a creative commons attribution.
We discuss two classes of bias that arise in casecontrol studies, selection bias and information bias. Selection bias occurs when the subjects studied are not representative of the target population about which conclusions are to be drawn. Bias in selection of cases cases are not derived from a well defined study base or source population bias in selection of controls controls should provide an unbiased sample of the exposure distribution in the study base control selection is a more important issue than case selection. What is bias and how can it affect the outcomes from research. Selection bias selection bias will occur as a result of the procedure used. In fact, wacholder and colleagues have extensively discussed aspects of design of case control studies and selection of controls in their article. Sep 12, 2011 we conclude that selection bias in case control selection of subjects from rare disease registries occurs and that this can be overcome through case control matching to minimize bias. We argue that our method is conceptually simpler than other commonly advocated methods and provides an intuitive way of performing sensitivity analysis. The next important point in designing a case control study is the selection of control patients. Mezei and kheifets 2006 show that selection bias in casecontrol studies can lead to overestimating the true odds ratio by up to a factor of 2. In a case control selection bias is when selection participation of cases and controls is related to exposure status. Selection bias in epidemiological studies teachepi.
Bias many forms of bias exist these can broadly be categorised as forms of. However, if present, such a bias would have wide implications for casecontrol studies in general. In theory, every case control study takes place within a cohort, although. Therefore, any problem with nested casecontrol studies must also be a problem for full cohort analysis. Original article a structural approach to selection bias miguel a. First, the effect of cigarette smoking on the risk of dementia is modified by age. A structural approach to selection bias semantic scholar.
In cohort studies, sampling of study participants is independent of the outcome. Adjusting for selection bias in retrospective, casecontrol. Several design options available in the planning stage of case control studies are examined. The aim of this paper is to address these issues for retrospective casecontrol studies. Selection bias and its implications for casecontrol.
Reducing selection bias in casecontrol studies from rare disease registries article pdf available in orphanet journal of rare diseases 61. If selection bias is suspected, there are circumstances under which it is possible to attempt to adjust for it. Mandep a synthesis of classical and recent thinking on the issues involved in selecting controls for case control studies is presented in this and two companion papers s. Selection bias can be introduced into case control studies with low response or participation rates if the likelihood of responding or participating is related to both the exposure and the outcome.
Because studies are carried out on people and have all the attendant practical and ethical constraints, they are almost invariably subject to bias. Those exposed have a greater sensitivity for recalling exposure reduced specificity specifically important in casecontrol studies when exposure history is obtained retrospectively cases may more closely scrutinize their past history looking for ways to explain their illness. The researchers are worried about the potential for recall bias since parents are being asked to recall what their. Retrospective casecontrol studies are more susceptible to selection bias than other epidemiologic studies as by design they require that both cases and controls are representative of the same. This chapter focuses on selection bias in case control studies. In a case control study selection bias occurs when subjects for the control group are not truly representative of the population that produced the cases. If the answer is yes, then selection bias is unlikel. It discusses control selection, evaluation of selection bias in casecontrol studies, and integrated assessment of potential for selection bias in casecontrol studies. Some examples of selection bias in epidemiology are. Topics covered include matching, control case ratio, choice of nested case control or case cohort design, twostage sampling, and other methods that can be used for control selection. Cohort studies smoking harmful in studies that enrolled younger subjects, and appeared protective in studies that enrolled the oldest subjects two possible explanations. In a casecontrol selection bias is when selectionparticipation of cases and controls is related to exposure status. Since most of the mfchildhood leukaemia epidemiological studies were casecontrol studies it has been proposed that selection bias may be fully or, at least, partially. Generally, the only disadvantages to nested casecontrol studies are the reduced precision and power due to sampling of controls, and the possibility of flaws in the sampling design or its implementation.
Alarge number of different sources and possible mechanisms of. Several of the methods in table 1 have been developed or derived from one another. We argue that our method is conceptually simpler than other commonly advocated methods and provides an. Introduction to study designs casecontrol studies health. Tools for evaluating the potential for selection bias in case control studies include comparing measured exposure prevalence among controls to an external population and determining whether the exposure among controls follows expected patterns, examining exposuredisease associations in relation to markers of susceptibility to bias, adjusting.
Casecontrol studies measure the probabilities of having an exposure or characteristics in. Dags y w s denote by wan exposure of interest and by y a disease under study. Therefore, application of this technique permits the study of treatment outcomes or natural history within rare disease registries. Methods to quantify selection bias are introduced together with analytical strategies to adjust for the bias including controlling for covariates. Case control studies also are used for diseases that have long latent periods long durations between exposure and disease manifestation and are ideal when multiple potential risk factors are at play. The impleme ntation of a method to reduce selection bias may also be viewed by researchers as an undesirable feature of their.
Uncovering selection bias in casecontrol studies using. Therefore, an appreciation of potential sources of bias has becomea critical issue in epidemiology. Further, by applying it to a casecontrol study, we show that our method can. Assessing bias in casecontrol studies proper selection of cases and controls kim suttontyrrell, drph casecontrol studies are retrospective investigations in which a diseased group cases and a diseasefree group controls are compared with the aim of uncovering risk factors that differ between the groups. Radiologist aware of patients smoking status when reading. Selection bias unc gillings school of global public health. Casecontrol studies are typically prone to selection bias g is true. Selection bias patients included in the study are not representative of the population to which the results will be applied. All cases should have an equal chance of being included in a case control study. In casecontrol studies the frequency of the exposure of interest is compared between a group of diseased subjects and a group of controls to determine whether an association exists between disease and exposure. Adjusting for selection bias in retrospective, casecontrol studies. Another form of measurement related bias in casecontrol studies is protopathic reverse causality bias.
Casecontrol studies are often populationbased, an important factor in avoiding many kinds of ascertainment bias. Sources of bias 1 selection bias ideally, control group has to be exactly the same as the group of cases except for the presence of disease selection process should ensure that both cases and controls are likely to come from a similar population selection bias occurs when the persons in one group are. Although there might not always be an entire airforce on the line when it comes to getting it right, its still essential for good research. In a casecontrol study selection bias occurs when subjects for the control group are not truly representative of the population that produced the. Mandep a synthesis of classical and recent thinking on the issues involved in selecting controls for casecontrol studies is presented in this and two companion papers s. How to investigate and adjust for selection bias in cohort. Pdf reducing selection bias in casecontrol studies from. The case control case referent design is really an efficient sampling technique for measuring exposuredisease associations in a cohort that is being followed up or study base all casecontrol studies are done within some cohort defined or not in reality, the distinction between cohort and casecontrol designs is artificial. It occurs when the exposure status of cases or controls influences the likelihood that they are entered into the study. Casecontrol studies are therefore placed low in the hierarchy of evidence. The term selection bias encompasses various biases in epidemiology. Selection bias selection bias will occur as a result of the procedure used to select study participants when the selection probabilities of exposed and unexposed cases and controls from the target population are differential and not proportional. Thus, some participants have the outcome of interest referred to as cases, whereas others do.