In that case, it is easier that exposed or unexposed subjects would be related to the result of interest, causing selection bias. What is bias and how can it affect the outcomes from research. Get a printable copy pdf file of the complete article 1. Bias in design this bias occurs when the case group and control group are not properly matched, and the confounding factors are not properly accounted for at the time of analysis. Unmatched case control study ii calculations of odds ratios in a stratified analysis age exposed cases controls odds ratios yes 9 8 types of information bias nondifferentialmisclassification between groups is approximately equal differentialamount of misclassification differs between groups more details in the confounding and bias in case control studies 28. Find out how cases are compared with controls in this type of study. In a casecontrol study selection bias occurs when subjects for the control group are not truly representative of the population that produced the. Selection bias in case control studies may occur when. Performance bias refers to systematic differences between groups that occur during the study. An investigation of report bias in a case control study of pregnancy outcome. Assistant professor institute of clinical pharmacy and pharmaceutical sciences, national cheng kung university 30 th annual meeting of the international society for pharmacoepidemiology taipei, taiwan october 23, 2014 1. Selection bias selection bias can result when the selection of subjects into a study or their likelihood of being retained in the study leads to a result that is different from what you would have gotten if you had enrolled the entire target population. Diagnoses case selection may be influenced by physicians knowledge of exposure example. Therefore, an appreciation of potential sources of bias has becomea critical issue in epidemiology.
Design and analysis of casecontrol studies uc davis health. The goal of a casecontrol study is the same as that of cohort studies, i. Introduction to study designs casecontrol studies health. Interviewer bias is more likely when disease status is known to interviewer. For a given disease, a casecontrol study can investigate multiple exposures. The healthy worker ef fect is a type of selection bias that may occur in occupational exposure studies when the exposed cases are workers but the non exposed study par ticipants controls are not workers. Case control studies are often populationbased, an important factor in avoiding many kinds of ascertainment bias. Bias bias occurs when there is a systematic difference between the results from a study and the true state of affairs bias is often introduced when a study is being designed, but can be introduced at any stage appropriate statistical methods can reduce the effect of bias, but may not eliminate it totally. A case control study, like other medical research, can help scientists find new medications and treatments. Better recall of exposure among the cases in a casecontrol study can result in a information bias b confounding c investigator bias d selection bias 37. Selection bias controls are used to provide an estimate of the exposure rate in the population. Another form of measurement related bias in casecontrol studies is protopathic reverse causality bias. In the design of case control studies, matching is a technique that is used to prevent. Adjusting for selection bias in retrospective, case.
Alarge number of different sources and possible mechanisms of. Due to the retrospective nature of casecontrol studies, they are particularly susceptible to the effects of bias, which may be introduced as a result of a poor study design or during the collection of exposure and outcome data. If selection bias is suspected, there are circumstances under which it is possible to attempt to adjust for it. Case studies are super useful for inspiration and ideas for new projects. Case control research subjects chosen based on disease status and assessed for previous illness to a risk factor of interest.
Pdf strengths and weaknesses of case control and cross. In these situations a casecontrol design offers an alternative that is much more efficient. 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 not representative of corresponding noncases in the study. In general, working individuals are healthier than non. 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. It costs relatively less and can be conducted in a shorter time for a given disease, a case control study can investigate multiple exposures when the real exposure is not known a case control study is. Remember that in a case control study the controls are used to estimate the exposure distribution i. The case control is a type of epidemiological observational study. Types of bias include selection bias, detection bias, information observation bias, misclassification, and recall bias. Box 1 example of recall bias in a case control study. But remind yourself all the time that only success stories are published. Casecontrol study an overview sciencedirect topics. Confounding and bias in casecontrol studies, chinglan cheng. Types of cases used in case control studies prevalent.
The researchers are worried about the potential for recall bias since parents are. Case control studies a case control study involves two populations cases and controls and has three distinct features. There are also different ways that cases can be identified, such as using populationbased cases or hospitalbased cases. A case control study is usually conducted before a cohort or an experimental study to identify the possible etiology of the disease. Mezei and kheifets 2006 show that selection bias in case control studies can lead to overestimating the true odds ratio by up to a factor of 2. Better recall of exposure among the cases in a casecontrol study can result in a information bias b confounding c investigator bias d selection bias 38. Analysis of casecontrol studies the odds ratio or is used in case control studies to estimate the strength of the association between exposure and outcome. The quality of the data is therefore determined to a large extent on the patients ability to accurately recall past exposures.
Radiologist aware of patients smoking status when reading. Case control studies are typically prone to selection bias g is true. However, casecontrol studies employ a different sampling strategy that gives them greater efficiency. Selection bias examples also occurs before subjects are identified for study diagnostic or workup bias.
Bias limits validity the ability to measure the truth within the study design and generalizability the ability to confidently apply the results to a larger population of study results. Bias in research joanna smith,1 helen noble2 the aim of this article is to outline types of bias across research designs, and consider strategies to minimise bias. The aim of this paper is to address these issues for retrospective case control studies. An example of this would be a patient with buergers disease enrolled in a case control study which attempts to retrospectively identify risk factors. 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. Selection bias occurs when the persons in one group are different. 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. 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. 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. Recall bias occurs when participants in a study are systematically more or less likely to recall and relate information on exposure depending on their outcome status, or to recall information regarding their outcome dependent on their exposure. It uses a control or comparison group to support or refute an inference.
Limitations and biases in cohort studies intechopen. In a case control study data on exposure is collected retrospectively. 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. Selection bias may also be introduced when exposed cases are. On the other hand, a prospective cohort design could be affected by the loss of followup. Information on known or suspected confounding characteristics is collected to evaluate and control confounding during the analysis. Crosssectional study is appropriate when we are interested in the prevalence of a disease or certain feature, cohort study provides answer to the incidence or relative risk, whereas case control. In case control studies, controls should be drawn from the same population as the cases, so they are representative of the population which produced the cases. Statistical bias types explained with examples part1. Epidemiologic study designs descriptive studies seeks to measure the frequency of disease andor collect descriptive data on risk factors analytic studies tests a causal hypothesis about the etiology of disease experimental studies compares, for example, treatments. Because case control studies focus on women who have given birth to a baby with birth defects, case control studies are usually far more statistically powerful than populationbased cohort studies of an equivalent size. Selection bias unc gillings school of global public health. Case control and cohort studies are observational studies that lie near the middle of the hierarchy of evidence.
However, casecontrol studies are less adept at showing a causal relationship than cohort studies. Both types of cohort studies may be influenced by information bias, confusion or interaction. 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. The estimates are not biased by differential loss to. Cohort studies and case control studies are two primary types of observational studies that aid in evaluating associations between diseases and exposures.
This work is licensed under a creative commons attribution. Recall bias may occur when the information provided on exposure differs between the cases and controls. Occurs when we place too much attention on information, even when it is not strictly relevant. For example, if participants know that they are in the active treatment rather than the control condition, this could create positive expectations that have an impact on treatment outcome beyond that of the intervention itself. Google scholar werler mm, pober br, nelson k, holmes lb. In case control studies, if exposure influences the diagnosis of the disease, detection bias occurs. Statistical bias types explained with examples part 1. We discuss two classes of bias that arise in case control studies, selection bias and information bias. Confounding and bias in casecontrol studies chinglan cheng, ph. Casecontrol studies are typically prone to selection bias gis.
An observational study is a study in which subjects are not randomized to the exposed or unexposed groups, rather the subjects are observed in order to determine both their exposure and their outcome status and the exposure status is thus not determined by the researcher. This is a particular problem for any study that requires an individual to recall their exposure to a possible risk, so case control studies are particularly vulnerable to this type of bias. A third source of bias, confounding, is not considered in this article. These types of studies, along with randomised controlled trials, constitute analytical studies, whereas case reports and case series define descriptive studies 1. Several factors have been found to influence publication, the most important being statistical significance, size of the study, funding, prestige, type of design, and study quality. Case control study outcome is pulmonary disease, exposure is smoking. Note that it is not possible to estimate the incidence of disease from a case control study unless the study is population based and all cases in a defined population are obtained. As a researcher, one can face the dilemma of choosing between many different types of research or studies designs, to determine which one will be the more suitable for the kind of topic we are investigating. Both exposure and outcome have occurred before the start of the study. This form of bias can be a particular problem in case control studies.