Case-control studies determine if there is an association between an exposure and a specific health outcome.

A case–control study compares a group of people with a disease to a group of people without the disease.

Case control studies proceed from effect (e.g. health outcome, condition, disease) to cause (exposure).

Case-control studies assess whether exposure is disproportionately distributed between the cases and controls, which may indicate that the exposure is a risk factor for the health outcome under study.

Case-control studies are frequently used for studying rare health outcomes or diseases.

 

 

 

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It is used to identify and assess factors that are associated with diseases or health conditions.

For example, patients with lung cancer had higher odds of smoking history than those without lung cancer.

 

A case-control study starts at the end of the process, when the outcome is already determined.

The investigator of a case-control study samples persons with the outcome of interest (cases) and constructs a comparison group (controls) of individuals who do not have the outcome of interest.

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[Unlike cohort or cross-sectional studies, subjects in case-control studies are selected because they have the health outcome of interest (cases). Selection is not based on exposure status. Controls, persons who are free of the health outcome under study, are randomly selected from the population out of which the cases arose.

The case-control study aims to achieve the same goals (comparison of exposed and unexposed) as a cohort study but does so more efficiently, by the use of sampling. After cases and controls have been identified, the investigator determines the proportion of cases and the proportion of controls that have been

 

Illustration of the design of a case-control study. Shaded areas represent exposed persons, and unshaded areas represent unexposed persons.

Image not available.

In essence, the study starts on the right-hand side of the diagram with the identification of cases and controls. It then “looks backward in time” to identify earlier exposure patterns among the subjects. If it turns out that more cases than controls had an exposure of interest, it may be evidence that exposure is associated positively with outcome (i.e., exposure increases the risk of disease development). However, if cases are less likely than controls to have had the exposure of interest, it may constitute evidence that exposure is associated inversely with outcome (i.e., exposure decreases the risk of disease development).1

Cases are those that have or have had the outcome. It is backward-looking.

Controls are those that lack the outcome. Controls are selected at random from the population as the cases. The principle that guides the selection of controls is that they must be representative of the underlying population that generated the cases. At a minimum, this means the controls must have been eligible to have had the outcome of interest.

Once the cases and controls have been selected, the prior exposure status of each study member is ascertained; then, defining the difference (see:Defining Differences of Entities) is used to determine whether or not the exposure rates differ between the case group and the control group.

There are multiple advantages of this design. Most case-control studies are modestly sized and can be completed relatively quickly. Because there is no need to wait for events to occur, the major time costs incurred are those related to identifying controls and interviewing selected cases and controls about their exposure status. The case-control study design is also well suited to the study of rare outcomes. Rare types of cancer, for example, may be difficult to observe in any given cohort, but a registry of patients who developed that cancer might be a suitable starting point for a case-control study.

 

The challenges of the case-control design are:

1. Iidentifying a representative sample of controls can be difficult and expensive. A more convenient control group might be available, but its exposure rate might not reflect the baseline rate in the population that generated the cases.

2. Asking study participants to recall details about past exposure can be problematic, especially if the exposure was not recent. This introduces the possibility of differential recall, if cases have given more thought about their potential exposures than controls have.

An example of a case-control study in the cardiology literature comes from Christiansen et al., who examined whether glucocorticoid use was associated with incident atrial fibrillation (3). Their cases, selected from hospital records, were patients who had had an inpatient diagnosis of incident atrial fibrillation. Controls were matched for age and sex. With more than 20,000 cases and 200,000 controls, this large case-control study was made possible through the use of population-based medical care databases in Denmark. Exposure to glucocorticoids was ascertained by searching pharmacy databases. Because these pharmacy data already existed for both cases and controls, this study could avoid some of the ascertainment bias present in typical epidemiological case-control studies that rely on self-reports of prior exposures. In the end, the authors found that the risk of atrial fibrillation nearly doubled with the use of glucocorticoids.

For example, a case–control study of asthma in high school students may identify a group of students who suffer from asthma and compare them to a control group of students without asthma in regard to factors such as presence of carpets in the house, presence of household pets and family history of asthma.

 

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Odds Ratio, Confidence Interval and p values are the statistical parameters that are used.


If the odds of exposure to one of these factors are different in the cases and controls, then
that factor is associated with asthma and may influence the occurrence of the disease.

 


Identification ofientific Research is creative, critical and systematic thinking. A research question points to the existence of an unknown  - to some area of human ignorance or uncertainty. It involves a systematic process that leads to the generation of new knowledge, refinement of knowledge, or extension of knowledge that contributes to the discipline of Medicine.

Cohort and case–control studies generally involve an extended period of time defined by the point when the study begins and the point when it ends; some process occurs, and a certain amount of time is required to assess it. For this reason, both cohort and case–control studies are sometimes also called longitudinal studies.

 


 


A case–control study begins with a group of cases of a specific disease or condition. A group of
people without that disease or condition is selected as control, or comparison, subjects. The
investigator then seeks to compare cases and controls with respect to previous exposures to factors
of interest. Information about prior exposure may be obtained by a variety of methods,
including self-administered questionnaires, interviews and medical examinations. Because in
case–control studies, information about exposure is generally collected after the disease has
already occurred, these studies are sometimes called retrospective studies.


Case–control studies start with the outcome and look backward for the exposure, unlike cohort
studies, which start from the exposure and look forward for the outcome. For example, a
case–control study of asthma in high school students may identify a group of students who suffer
from asthma and compare them to a control group of students without asthma in regard to factors
such as presence of carpets in the house, presence of household pets and family history of asthma.
If the odds of exposure to one of these factors are different in the cases and controls, then
that factor is associated with asthma and may influence the occurrence of the disease.

Another example: In case-control efficacy studies, vaccination status of persons with disease is compared with vaccination status of healthy controls. The number of vaccinated and unvaccinated cases and controls is included in a two-by-two table, and vaccine efficacy is calculated as 1 minus the odds ratio (VE (%) = (1 − OR) × 100). When the proportion of cases who have been vaccinated is less than the proportion of vaccinated controls, the odds ratio is <1 and the point estimate for efficacy indicates that immunization is protective. The precision of the estimate is expressed by the 95% confidence interval. A lower 95% confidence limit that is greater than 0% indicates statistically significant protection; often investigators set power of vaccine efficacy studies so that the lower confidence limit is much greater than zero and consistent with meaningful levels of protection. The most important component of a case-control efficacy study is selecting controls who have the same opportunity for immunization as do cases. If cases had less opportunity to be immunized, results will be biased toward showing protection. Factors such as low socioeconomic status, which may increase the risk of disease and decrease the chance of being immunized, are potential confounding variables and can be controlled for by matching controls to cases for those factors.

Bias and errors

 

  • 1

    Recall bias: a risk for retrospective studies. When patients cannot remember things, they may inadvertently overestimate or underestimate risk factors. For example, John died of lung cancer, and his angry wife remembers him smoking “like a chimney,” whereas Mike died of a non–smoking-related disease, and his loving wife denies that he smoked much. In reality, both men smoked one pack per day.

  • 2.

    Interviewer bias: occurs when there is no blinding. When a scientist receives big money to do a study and wants to find a difference between cases and controls, he or she may inadvertently interpret the same patient comment or outcome as “not significant” in the control group and “significant” in the treatment group.

  • 3. Unacceptability bias: Patients do not admit to embarrassing behavior. They claim to exercise more than they do to please the interviewer, or they may claim to take experimental drugs when they spit them out.
    Odds ratios are most commonly used in case-control studies
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  • A case control study is associated with the Odds Ratio (OR).

    Reference: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2938757/ (Note correction to article)