Remember that in a case control study the controls are used to estimate the exposure distribution i. Thus, some participants have the outcome of interest referred to as cases, whereas others do. In a casecontrol selection bias is when selectionparticipation of cases and controls is related to exposure status. Pdf reducing selection bias in casecontrol studies from. We argue that our method is conceptually simpler than other commonly advocated methods and provides an. The term selection bias encompasses various biases in epidemiology.
Cases are often recruited using convenience samplingfor example, from hospital clinics, which are easily accessible. Some are also aimed at particular study designs, for example two were developed specifically for case control studies 5 7. Adjusting for selection bias in retrospective, casecontrol studies. 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. Dags y w s denote by wan exposure of interest and by y a disease under study. Another form of measurement related bias in casecontrol studies is protopathic reverse causality bias. The primary challenge in designing a casecontrol study is the appropriate selection of cases and controls. 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.
The impleme ntation of a method to reduce selection bias may also be viewed by researchers as an undesirable feature of their. 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. Selection bias controls are used to provide an estimate of the exposure rate in the population. Casecontrol studies are often populationbased, an important factor in avoiding many kinds of ascertainment bias. However, if present, such a bias would have wide implications for case control studies in general. This chapter focuses on selection bias in case control studies. A casecontrol study is usually conducted before a cohort or an experimental study to identify the possible etiology of the disease. 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.
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. In this design, participants are selected for the study based on their outcome status. Selection bias in casecontrol studies oxford scholarship. Unbiased selection of cases and controls is very crucial to this research design.
Several design options available in the planning stage of case control studies are examined. 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. Casecontrol studies measure the probabilities of having an exposure or characteristics in. This chapter focuses on selection bias in casecontrol studies. Several of the methods in table 1 have been developed or derived from one another.
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. Selection bias patients included in the study are not representative of the population to which the results will be applied. This can occur when exposure status influences selection. Some examples of selection bias in epidemiology are. The researchers are worried about the potential for recall bias since parents are being asked to recall what their. All cases should have an equal chance of being included in a case control study. Jan 30, 2015 casecontrol studies are typically prone to selection bias g is true.
If selection bias is suspected, there are circumstances under which it is possible to attempt to adjust for it. In a casecontrol study selection bias occurs when subjects for the control group are not truly representative of the population that produced the. There are several types of selection bias, and most can be prevented before the results are delivered. Uncovering selection bias in casecontrol studies using. Methods to quantify selection bias are introduced together with analytical strategies to. 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.
Adjusting for selection bias in retrospective, casecontrol. It discusses control selection, evaluation of selection bias in casecontrol studies, and integrated assessment of potential for selection bias in casecontrol studies. The next important point in designing a case control study is the selection of control patients. An example of the role of selection bias is given in the worked example of casecontrol studies. Several design options available in the planning stage of casecontrol studies are examined. Selection bias and its implications for casecontrol.
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. Methods to quantify selection bias are introduced together with analytical strategies to adjust for the bias including controlling for covariates. One of the most significant triumphs of the casecontrol study was the demonstration of the link between tobacco smoking and lung cancer, by richard doll and bradford hill. Alarge number of different sources and possible mechanisms of. In theory, every case control study takes place within a cohort, although. 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. Bias in full cohort and nested casecontrol studies. 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. 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.
Selection bias occurs when the subjects studied are not representative of the target population about which conclusions are to be drawn. 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. Avoiding selection bias is a particular challenge in the design of case control studies. This work is licensed under a creative commons attribution. However, if present, such a bias would have wide implications for casecontrol studies in general. Selection bias selection bias will occur as a result of the procedure used. 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. Casecontrol studies are typically prone to selection bias gis. 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. The primary challenge in designing a case control study is the appropriate selection of cases and controls. Jul 15, 2005 we discuss two classes of bias that arise in casecontrol studies, selection bias and information bias. A third source of bias, confounding, is not considered in this article. 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.
First, the effect of cigarette smoking on the risk of dementia is modified by age. We argue that our method is conceptually simpler than other commonly advocated methods and provides an intuitive way of performing sensitivity analysis. Reducing selection bias in casecontrol studies from rare disease registries article pdf available in orphanet journal of rare diseases 61. Retrospective casecontrol studies are more susceptible to selection bias than other epidemiologic studies as by design they require that both cases and. 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. Selection bias in epidemiological studies teachepi. 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. What is bias and how can it affect the outcomes from research. The aim of this paper is to address these issues for retrospective casecontrol studies. We discuss two classes of bias that arise in casecontrol studies, selection bias and information bias. Diagnoses case selection may be influenced by physicians knowledge of exposure example. In a cohort study selection bias is when selectionparticipation of the exposed and unexposed subjects are related to disease status. The authors have conducted a casecontrol study to evaluate the effect of various oha on bone mineral density bmd in patients with t2dm. 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.
In this poster we use directed acyclic graphs1 to model selection bias. It occurs when the exposure status of cases or controls influences the likelihood that they are entered into the study. Selection bias can present a serious problem for valid odds ratio estimation in casecontrol studies as demonstrated. Because studies are carried out on people and have all the attendant practical and ethical constraints, they are almost invariably subject to bias. In contrast, in casecontrol studies participants are sampled at different rates. Original article a structural approach to selection bias miguel a. The effect of potential problems in obtaining comparable accuracy. Therefore, any problem with nested casecontrol studies must also be a problem for full cohort analysis.
Selection bias and its implications for casecontrol studies. 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. 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. In cohort studies, sampling of study participants is independent of the outcome. In fact, wacholder and colleagues have extensively discussed aspects of design of case control studies and selection of controls in their article. Casecontrol studies are typically prone to selection bias g is true. Selection bias in case control studies may occur when. 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. Adjusting for selection bias in retrospective, case. 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. 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.
We describe examples of selection bias in casecontrol studies eg, inappropriate selection of controls and cohort. Radiologist aware of patients smoking status when reading. Selection bias can present a serious problem for valid odds ratio estimation in case control studies as demonstrated. Bias many forms of bias exist these can broadly be categorised as forms of. Case control study outcome is pulmonary disease, exposure is smoking. Cohort studies smoking harmful in studies that enrolled younger subjects, and appeared protective in studies that enrolled the oldest subjects two possible explanations. 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. Selection bias occurs if the recruited cases or controls are systematically different from the population of people they are intended to represent. 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 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. Casecontrol study an overview sciencedirect topics. In a case control selection bias is when selection participation of cases and controls is related to exposure status. 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.
Casecontrol study design is a type of observational study. 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. 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. If the answer is yes, then selection bias is unlikel. 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. Casecontrol studies are therefore placed low in the hierarchy of evidence. It discusses control selection, evaluation of selection bias in case control studies, and integrated assessment of potential for selection bias in case control studies. 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. 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. Reducing selection bias in casecontrol studies from rare. 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. Therefore, application of this technique permits the study of treatment outcomes or natural history within rare disease registries.
In a cohort study selection bias is when selection participation of the exposed and unexposed subjects are related to disease status. 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. Further, by applying it to a casecontrol study, we show that our method can. A structural approach to selection bias semantic scholar.
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. Therefore, any problem with nested case control studies must also be a problem for full cohort analysis. Introduction to study designs casecontrol studies health. Selection bias unc gillings school of global public health. 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. How to investigate and adjust for selection bias in cohort. 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. We call for better reporting and for evaluation of the potential for selection bias in all case control studies, as well as, for the development of novel methods in control selection and recruitment.
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