Choice of controls in case control studies pdf

Mandel2 types of control groups are evaluated using the principles described in paper 1 of the series, selection of controls in casecontrol studies s. Introduction a casecontrol study is designed to help determine if an exposure is associated with an outcome i. To learn when it is ideal to use a case control study design 2. Selection bias will occur in cohort studies if the rates of. This paper will explore the concepts of the choice of controls in observational analytic epidemiological case control studies.

Controls can be taken from known or unknown study populations. The odds ratio is the measure of association for a casecontrol study. Four case control studies implicating smoking and lung cancer appeared in 1950, establishing the method in epidemiology. The goal of cohort studies is to test an association, but case control studies just document the frequency of risk factors. In view of this there are two key principles that should be followed in selecting controls. Analysis of casecontrol studies the odds ratio or is used in casecontrol studies to estimate the strength of the association between exposure and outcome. A case control study is designed to help determine if an exposure is associated with an outcome i. Case control studies have been used to estimate the odds ratios ors for involvement in road traffic accidents. Random sampling from the study base, where controls are chosen independently of characteristics of the cases, is the simplest strategy for. Design, conduct, and analysis of case control studies christy mckinney, phd, mph. The goal is to retrospectively determine the exposure to the risk factor of interest from each of the two groups of individuals. This work is licensed under a creative commons attribution.

The above study, dating from more than 40 years ago, contrasts with larger scale more recent studies, such as the casecontrol study of smokerrelated deaths in india. While cohort studies are sampled according to exposure, characteristic or cause, casecontrol studies are sampled according to disease or outcome. Casecontrol studies e r i c n o t e b o o k s e r i e s casecontrol studies are used to determine if there is an association between an exposure and a specific health outcome. Introduction to study designs casecontrol studies health. Several design options available in the planning stage of case control studies are examined. Give the rationale for the choice of cases and controls participants 6 b for matched studies, give matching criteria and the number of controls per case. Casecontrol studies have been used to estimate the odds ratios ors for involvement in road traffic accidents. The identification of the appropriate study base from which to select controls is the primary challenge in the design of case control studies. Case control studies declare a specific outcome of interest i. First, identify the cases a group known to have the outcome and the controls a group known to be free of the outcome. A casecontrol study is usually conducted before a cohort or an experimental study to identify the possible etiology of the disease. Cases and controls are registered in a cohort study. In these situations a cohort study is generally prohibitively expensive or the time frame of the study required for data collection is. In a nested casecontrol, a control is chosen randomly from among the subjects that are still diseasefree at the time each case got the disease as in a casecohort, a control matched to a previous case can later turn out to be a case himself.

First, identify the cases a group known to have the outcome and the controls a group known to be. Starting with an outcome like disease, a casecontrol study looks backwards in time for exposures that might have caused the outcome. While cohort studies are sampled according to exposure, characteristic or cause, case control studies are sampled according to disease or outcome. As discussed in the previous chapter, one of the drawbacks of using a longitudinal approach to investigate the causes of disease with low incidence is that large and lengthy studies may be required to give adequate statistical power. Casecontrol study an overview sciencedirect topics.

In casecontrol studies where cases are hospital based, it is common to recruit controls from the hospital population. In theory, the case control study can be described simply. Cohort, cross sectional, and casecontrol studies are collectively referred to as observational studies. Unit 4 practice questions university of south florida. Selection of controls in casecontrol studies american. Selection bias unc gillings school of global public health.

Case control studies trace backwards from outcome to exposure. In theory, the casecontrol study can be described simply. In theory, every case control study takes place within a cohort, although in practice it can be difficult to characterize the cohort or study base. Summary the statistical efficiency of a study design which matches each case with k controls is compared with. However, not infrequently it is difficult or expensive to obtain case observations while suitable control observations present no such problems. Several design options available in the planning stage of casecontrol studies are examined. Number of controls per case increases statistical efficiency power ability to detect associations 4. Topics covered include matching, control case ratio, choice of nested case. Thus, casecontrol studies can be thought of as occurring in the midst of a larger cohort study nested casecontrol studies12 being a nice example. Topics covered include matching, controlcase ratio, choice. The study aimed to determine whether hospital controls could be used in case control studies with minimal bias, where resource constraints limit recruitment of community controls. Modelbased clustering for identifying diseaseassociated. Rodrigues l, kirkwood br 1990 casecontrol designs in the study of common diseases.

However, it actually only applies to those case control studies in which controls are sampled only from the nondiseased rather than the whole population. Unmatched casecontrol studies are typically analysed using the mantelhaenszel method10 or unconditional logistic regression. When case control studies were first developed, most were conducted retrospectively, and it is sometimes assumed that the raredisease assumption applies to all case control studies. Pdf case control studies in medical research researchgate. Often these studies are the only practicable method of studying various problems, for example, studies of aetiology, instances where a randomised controlled trial might be unethical, or if the condition to be studied is rare. Apr 16, 2005 use of control comparison groups is a powerful research tool. A known group consists of a defined population observed over a period, such as passengers on a cruise ship. Cases and controls are patients who are hospitalized or outpatients.

In our second paper 1, we apply the principles presented in this paper to the selection of control groups used in casecontrol studies, including population controls, hospital controls, medical practice controls, friend controls, and relative controls. Case control studies e r i c n o t e b o o k s e r i e s case control studies are used to determine if there is an association between an exposure and a specific health outcome. One may start with a group of people with a known exposure and a comparison group control group without the exposure and follow them through time to see what outcomes result, but this does not constitute a casecontrol study. Internal controls case studies government oversight. Begin with the outcome and look for features of people who share that outcome, then compare characteristics with subjects who do not. However, the choice of controls from a hospital setting should not include individuals with an outcome related to the exposures being studied. The study aimed to determine whether hospital controls could be used in casecontrol studies with minimal bias, where resource constraints limit recruitment of community controls. Internal controls case studies patrick cogley regional inspector general for audit services, kansas city debra keasling assistant regional inspector general for. 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. Cohort, cross sectional, and case control studies are collectively referred to as observational studies. 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. In this paper, a theoretical framework for selecting controls in casecontrol studies is developed.

It quantifies the relationship between an exposure such as eating a food or attending an event and a disease in a casecontrol study. The general reasoning in the design of case control studies has been that the investiga tor chooses samples from potential cases and controls, that the null hypothesis refers to the prevalence of etiologic exposure in these two groups, and that the cases and controls should be as similar as possible, except for factors relating to exposure. It therefore seems of interest to find out how much statistical efficiency can be gained by matching each case with several independent controls. Cole1 identified the selection of the control group as. Casecontrol studies trace backwards from outcome to exposure. Evidence shows that for hospitalbased case control studies, community controls are the best choice if high response rates can be attained neupane et al. On choosing the control group in casecontrol studies. What should be the ratio of cases to controls in case. It must be clearly stated prior to the design of the casecontrol study. Epidemiologic study designs johns hopkins hospital. Casecontrol studies rachel walden, mlis eskind biomedical library mla 2007. Or, in a casecontrol study of occupational risk factors using pop ulation controls, investigators might con sider excluding cases diagnosed at. Topics covered include matching, controlcase ratio, choice of nested case.

In case control studies subjects are selected and grouped based on their disease status, but in cohort studies subjects are selected and grouped based on exposure status. Selection of controls in casecontrol studies american journal of. Ury medical methods research department, permanente medical group, 3779 piedmont ave. In a casecohort, the controls are a random sample of all study subjects at baseline. Nested casecontrol within a cohort study, compares all cases to a subset of persons who did not develop disease. 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. Note that it is not possible to estimate the incidence of disease from a casecontrol study unless the study is population based and all cases in a defined population are obtained. As noted above, it is always useful to think of a casecontrol study as being nested within some sort of a cohort, i. Internal controls case studies patrick cogley regional inspector general for audit services, kansas city debra keasling assistant regional inspector general for audit services, denver. One group was selected from subjects presenting to the heath unit the case attended for routine. Casecontrol studies declare a specific outcome of interest i. Updates on the demise of the rare disease assumption and the choice of sampling scheme for controls.

Casecontrol studies begin at the end outcome is the basis to select the comparison groups they test hypotheses concerning the association and magnitudes of a relationship between outcomes and exposures the strength of association is measured by an odds ratio or which is a good proxy for a relative risk when disease is rare. The goal of cohort studies is to test an association, but casecontrol studies just document the frequency of risk factors. Cohort studies and case control studies are two primary types of observational studies that aid in evaluating associations between diseases and exposures. Sep 23, 2019 genomewide association studies gwass aim to detect genetic risk factors for complex human diseases by identifying diseaseassociated singlenucleotide polymorphisms snps. Design, conduct, and analysis of case control studies. A casecontrol study is designed to help determine if an exposure is associated with an outcome i. In casecontrol studies, controls estimate the frequency of an exposure in the population under study. 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. Compared to prospective cohort studies, casecontrol study tends to be less.

That choice affects the inferences that can be drawn from the trial, the ethical acceptability of the. Choice of controls for a casecontrol study in bangladesh. Casecontrol studies advantages good design for rare, chronic and long latency diseases relatively inexpensive population size and time allows for the examination of multiple exposures estimate odds ratios hospitalbased studies and outbreaks. In most case control studies the hardest decision is the choice of the control group, as in the ideal control group the proportion exposed is the same as in the population that produced the cases. Starting with an outcome like disease, a case control study looks backwards in time for exposures that might have caused the outcome. The choice of control group is always a critical decision in designing a clinical trial.

First, identify the cases a group known to have the outcome. Strengths and weaknesses of casecontrol studies strengths weaknesses ef. It is the most difficult and most controversial aspect of. By including tobacco consumption levels as well as alcohol consumption, for cases and controls, estimates of relative risks by age group can be obtained. When the study group is known, a sample of the population. In casecontrol studies subjects are selected and grouped based on their disease status, but in cohort studies subjects are selected and grouped based on exposure status. Genomewide association studies gwass aim to detect genetic risk factors for complex human diseases by identifying diseaseassociated singlenucleotide polymorphisms snps. An alternative which avoids this difficulty is the case control or case referent design. Selection of controls in casecontrol studies mcgill university. Evidence shows that for hospitalbased casecontrol studies, community controls are the best choice if high response rates can be attained neupane et al. 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.

The casecontrol study design is an excellent choice of study when the disease is rare, has a long induction period, the exposure data are difficult to obtain, or very little is known about the disease. Cohort studies and casecontrol studies are two primary types of observational studies that aid in evaluating associations between diseases and exposures. A comparison of two control groups in a case control study of the efficacy of bcg revaccination. The healthy worker effect is a type of selection bias that may occur in occupational exposure studies when the exposed. Use of control comparison groups is a powerful research tool.

Casecohort within a cohort study, compares all cases to a random subsample of the cohort subcohort can be used for multiple case groups. All studies which contain cases and controls are not casecontrol studies. To learn how selection bias can occur and some strategies to minimize it 3. Case control studies are observational because no intervention is attempted and no attempt is made to alter the course of the disease. Analysis of case control studies the odds ratio or is used in case control studies to estimate the strength of the association between exposure and outcome. Or stands for odds ratio and rr stands for relative risk. Hospital controls and community controls were compared for sociodemographic and risk factor variables in a study of smokeless tobacco slt use and coronary heart. That choice affects the inferences that can be drawn from the trial, the ethical acceptability of the trial, the degree to which bias in conducting and analyzing the study can be minimized, the. In case control studies, controls estimate the frequency of an exposure in the population under study. Mandel2 types of control groups are evaluated using the principles described in paper 1 of the series, selection of controls in case control studies s. A casecontrol study also known as casereferent study is a type of observational study in which two existing groups differing in outcome are identified and compared on the basis of some supposed causal attribute. Introduction of casecontrol study aug, 5, 2014 hirohide yokokawa, m.