Case Control Studies (2023)

Case Control Studies (1) Case Control Studies

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A case-control study is a type of observational study commonly used to look at factors associated with diseases or outcomes.[1] The case-control study starts with a group of cases, which are the individuals who have the outcome of interest. The researcherthen tries to construct a second group of individuals called the controls, who are similar to the case individuals but do not have the outcome of interest. The researcher then looks at historical factors to identify if some exposure(s) is/are found more commonly in the cases than the controls. If the exposureis found more commonly in the casesthan in the controls, the researcher can hypothesize that the exposuremay be linked to the outcome of interest.

For example, a researcher may want to look at the rare cancer Kaposi's sarcoma. The researcher would find a group of individuals with Kaposi's sarcoma (the cases) andcompare them to a group of patients who aresimilar to the cases in most ways butdo not have Kaposi's sarcoma (controls). The researcher could then ask about various exposures to see if any exposure is more common in those with Kaposi's sarcoma (the cases) than those without Kaposi's sarcoma (the controls). The researchermight find that those with Kaposi's sarcoma are more likely to have HIV, and thus conclude that HIV may be a risk factor for the development of Kaposi's sarcoma.


(Video) Cohort and Case Control Studies

There are many advantages to case-control studies. First, the case-control approachallowsfor the study of rare diseases. If a diseaseoccurs very infrequently,one would have to follow a large group of people for a long period of time toaccrueenoughincident cases to study. Such use of resources may be impractical, so a case-control study canbe usefulfor identifying current cases and evaluating historical associated factors. For example, if a diseasedeveloped in 1 in 1000 people per year (0.001/year) then in ten yearsone would expect about 10 cases of a disease to exist in a group of 1000 people. If the disease is much rarer, say 1 in 1,000,0000 per year (0.0000001/year) this would require either having to follow 1,000,0000 people for ten years or 1000 people for 1000 years toaccrue ten total cases. As it may be impractical to follow 1,000,000 for ten years or to wait 1000 years for recruitment,a case-control study allows for a more feasible approach.

Second, the case-control study design makes it possible to look at multiple risk factors at once. Inthe example above about Kaposi's sarcoma, the researcher could ask boththe cases and controls about exposures to HIV, asbestos, smoking, lead, sunburns, aniline dye, alcohol, herpes, human papillomavirus, or any number of possible exposures to identify those most likely associated with Kaposi's sarcoma.

Case-control studies can also be very helpful when disease outbreaks occur, and potential links and exposures need to be identified. This study mechanism can be commonly seen in food-related disease outbreaks associated with contaminated products, or when rare diseases start to increase in frequency, as has been seen with measles in recent years.

Because of these advantages, case-control studies are commonly used as one of the first studies to build evidence of an association between exposure and an event or disease.

In a case-control study, the investigator caninclude unequal numbers of cases with controls such as2:1 or 4:1 to increase the power of the study.

Disadvantages and Limitations

The most commonly cited disadvantage in case-control studies is the potential for recall bias.[2]Recall bias in a case-control study is the increased likelihood that those with theoutcomewill recall and report exposures compared to those without the outcome. In other words, even if both groups had exactly the same exposures, the participants in the cases group may report the exposure more often than the controls do. Recall bias may lead to concluding that there are associations between exposure and disease that do not, in fact, exist. It is due to subjects'imperfect memories of past exposures. If people with Kaposi's sarcoma are asked about exposure and history (e.g., HIV, asbestos, smoking, lead, sunburn, aniline dye, alcohol, herpes, human papillomavirus), the individuals with the disease are more likely to think harder about these exposures and recall having some of the exposures that the healthy controls.

Case-control studies, due to their typically retrospective nature, can be used to establish a correlationbetween exposures and outcomes, but cannot establish causation. These studies simply attempt to find correlations between past events and the current state.

When designing a case-control study, the researcher must find an appropriate control group. Ideally, the case group (those with the outcome) and the control group (those without the outcome) will have almost the same characteristics, such as age, gender, overall health status, and other factors.The two groups should have similar histories and live in similar environments. If, for example, our cases of Kaposi'ssarcomacame from across the country but our controls were only chosen from a small community in northernlatitudeswhere people rarely go outside or get sunburns, asking about sunburn may not be a valid exposure to investigate. Similarly, if all of the cases of Kaposi's sarcoma were found to come from a small community outside a battery factory with high levels of lead in theenvironment, then controls from across the country with minimal lead exposure would not provide an appropriate control group. The investigator must puta great deal of effort into creating a proper control group to bolster the strength of the case-control study as well as enhance their ability to find true and valid potential correlations between exposures and disease states.

Similarly, the researcher must recognize the potential for failing to identify confounding variables or exposures, introducing the possibility of confounding bias, which occurs when a variable that is not being accounted for that has a relationship with both the exposure and outcome. This can cause us to accidentally be studying something we are not accounting for but that may be systematically different between the groups.


The major method for analyzing results in case-control studies is the odds ratio (OR). The odds ratio is the odds of having a disease (or outcome) with the exposure versus the odds of having the disease without the exposure. The most straightforward way to calculate the odds ratio is with a 2 by 2 table divided by exposure and disease status (see below). Mathematically we can write the odds ratio as follows.

Odds ratio = [(Numberexposed with disease)/(Number exposed without disease)]/[(Number not exposed to disease)/(Number not exposed without disease)]

This can be rewritten as:

Odds ratio = [(Numberexposed with disease) x (Number not exposed without disease)] / [(Number exposed without disease) x (Number notexposedwith disease)]

(Video) Case-Control Studies: A Brief Overview

The odds ratio tells us how strongly the exposure is related to the disease state. An odds ratio of greater than one implies the disease is more likely with exposure. An odds ratio of less than one implies the disease is less likely with exposure and thus the exposure may be protective. For example, a patient with a prior heart attack taking a daily aspirin has a decreased odds of having another heart attack (odds ratio less than one). An odds ratio of one implies there is no relation between the exposure and the disease process.

Odds ratios are often confused with Relative Risk (RR), which is a measure of the probability of the disease or outcome in the exposed vs unexposed groups. For very rare conditions, the OR and RR may be very similar, but they are measuring different aspects of the association between outcome and exposure. The OR is used in case-control studies because RR cannot be estimated; whereas in randomized clinical trials, a direct measurement of the development of events in the exposed and unexposed groups can be seen. RR is also used to compare risk in other prospective study designs.

Issues of Concern

The main issues of concern with a case-control study are recall bias, its retrospective nature, the need for a careful collection of measured variables, and the selection of an appropriate control group.[3]These are discussed above in the disadvantages section.

Clinical Significance

A case-control study is a good tool for exploring risk factors for rare diseases or when other study types are not feasible. Many times an investigator willhypothesizea list of possible risk factors for a disease process and will then use a case-control study to see if there are any possible associations between the risk factors and the disease process. The investigator can then use the data from the case-controlstudy tofocus on a few of the mostlikelycausativefactors anddevelop additional hypotheses or questions. Then through further exploration, often using otherstudy types (such ascohortstudies or randomizedclinicalstudies) the researchermay be able todevelop further support for the evidence of the possible association between theexposure andthe outcome.

Enhancing Healthcare Team Outcomes

Case-control studies are prevalent in all fields of medicine from nursing and pharmacy to use in public health and surgical patients. Case-control studies are important for each member of the health care team to not only understand their common occurrence in research but because each part of the health care team has parts to contribute to such studies. One of the most important things each party provides is helping identify correct controls for the cases. Matching the controls across a spectrum of factors outside of the elements of interest take input from nurses, pharmacists, social workers, physicians, demographers, and more. Failure for adequate selection of controls can lead to invalid study conclusions and invalidate the entire study.

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Case Control Studies (2)
2x2 table with calculations for the odds ratio and 95% confidence interval for the odds ratio
Contributed by Steven Tenny MD, MPH, MBA

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(Video) Case control studies

Steven Tenny

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Connor C. Kerndt

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Mary R. Hoffman


3/28/2022 6:03:44 PM

PubMed Link:

Case Control Studies

(Video) 5. Case control studies



Methodology Series Module 2: Case-control Studies., Setia MS,, Indian journal of dermatology, 2016 Mar-Apr [PubMed PMID: 27057012]


Bias in observational study designs: case-control studies., Sedgwick P,, BMJ (Clinical research ed.), 2015 Jan 30 [PubMed PMID: 25636996]


Efficient sampling in unmatched case-control studies when the total number of cases and controls is fixed., Groenwold RHH,van Smeden M,, Epidemiology (Cambridge, Mass.), 2017 Jul 4 [PubMed PMID: 28682849]

(Video) Epidemiological Studies - made easy!


What is case-control studies in research? ›

(kays-kun-TROLE STUH-dee) A study that compares two groups of people: those with the disease or condition under study (cases) and a very similar group of people who do not have the disease or condition (controls).

What is a case-control study example? ›

For example, investigators conducted a case-control study to determine if there is an association between colon cancer and a high fat diet. Cases were all confirmed colon cancer cases in North Carolina in 2010. Controls were a sample of North Carolina residents without colon cancer. The odds ratio was 4.0.

What is a case-control study vs cohort study? ›

Whereas the cohort study is concerned with frequency of disease in exposed and non-exposed individuals, the case-control study is concerned with the frequency and amount of exposure in subjects with a specific disease (cases) and people without the disease (controls).

When would you use a case-control study? ›

Case-control studies are advantageous under the following circumstances:
  1. When exposure data are expensive or difficult to obtain, e.g., assessing pesticide levels in blood or other medical tests.
  2. When the disease has a long induction and/or latent period, e.g., cancer, dementia. ...
  3. When the outcome (disease) is rare.
21 Apr 2021

Is case-control study quantitative or qualitative? ›

In a health care context, randomised controlled trials are quantitative in nature, as are case-control and cohort studies. Surveys (questionnaires) are usually quantitative .

What are the benefits of a case-control study? ›

  • Good for studying rare conditions or diseases.
  • Less time needed to conduct the study because the condition or disease has already occurred.
  • Lets you simultaneously look at multiple risk factors.
  • Useful as initial studies to establish an association.

What are the 3 types of case studies? ›

Types of Case Studies
  • Collective case studies: These involve studying a group of individuals. ...
  • Descriptive case studies: These involve starting with a descriptive theory. ...
  • Explanatory case studies: These are often used to do causal investigations.
7 Nov 2022

What are the characteristics of a case-control study? ›

Case-control studies are retrospective, and cases are identified at the beginning of the study; therefore there is no long follow up period (as compared to cohort studies). Efficient for the study of diseases with long latency periods. Efficient for the study of rare diseases. Good for examining multiple exposures.

Are case-control studies observational? ›

Cohort studies and case-control studies are two primary types of observational studies that aid in evaluating associations between diseases and exposures.

What type of study design is a case-control study? ›

Case-Control study design is a type of observational study. In this design, participants are selected for the study based on their outcome status. Thus, some participants have the outcome of interest (referred to as cases), whereas others do not have the outcome of interest (referred to as controls).

Why is case-control better than cohort? ›

Cohort studies work well for rare exposures–you can specifically select people exposed to a certain factor. But this design does not work for rare diseases–you would then need a large study group to find sufficient disease cases. Case-control studies are relatively simple to conduct.

Why is it called a cohort study? ›

The term “cohort” refers to a group of people who have been included in a study by an event that is based on the definition decided by the researcher. For example, a cohort of people born in Mumbai in the year 1980. This will be called a “birth cohort.” Another example of the cohort will be people who smoke.

What are the advantages and disadvantages of case-control studies? ›

They are less costly and less time-consuming; they are advantageous when exposure data is expensive or hard to obtain.
  • They are subject to selection bias.
  • They are inefficient for rare exposures.
  • Information on exposure is subject to observation bias.
  • They generally do not allow calculation of incidence (absolute risk).
7 Jun 2016

What kind of bias are in case-control study? ›

The most commonly cited disadvantage in case-control studies is the potential for recall bias. Recall bias in a case-control study is the increased likelihood that those with the outcome will recall and report exposures compared to those without the outcome.

How do you analyze a case-control study? ›

Case-control studies produce the odds ratio to measure the strength of the link between exposure and the outcome. An odds ratio is the ratio of exposure probabilities in the case group to the odds of response in the control group. Calculating a confidence interval for each odds ratio is critical.

What are the 4 main types of research? ›

There are four main types of Quantitative research: Descriptive, Correlational, Causal-Comparative/Quasi-Experimental, and Experimental Research. attempts to establish cause- effect relationships among the variables.

Is a case-control study analytical or descriptive? ›

Analytical observational studies include case””control studies, cohort studies and some population (cross-sectional) studies. These studies all include matched groups of subjects and assess of associations between exposures and outcomes.

What is the difference between a case study and qualitative research? ›

It is more of a qualitative method of research where it understand complex issues by deeply observing and analyzing the event or situation by collecting and reporting the data related to the event or situation. Case study research is more towards description rather than immediate cause and effect finding.

What are 2 limitations of case-control studies? ›

The main limitations of case-control studies are:
  • 'Recall bias' When people answer questions about their previous exposure to certain risk factors their ability to recall may be unreliable. ...
  • Cause and effect. ...
  • 'Sampling bias' ...
  • Other limitations.

What are 3 disadvantages of case studies? ›

Limitations of Case Studies

Lacking scientific rigour and providing little basis for generalization of results to the wider population. Researchers' own subjective feeling may influence the case study (researcher bias). Difficult to replicate. Time-consuming and expensive.

What type of sample is a case study? ›

Purposeful sampling is generally used in case study research; therefore, explain sampling procedures and case selection, and the defining characteristics and typicality or atypicality of the case: Note whether the case in question is a deviant or extreme case, a critical case, a convenience case, a politically ...

How do you identify a case study? ›

A case study should include background information on the specific topic, an analysis of the case under student showing problems or effective strategies, as well as recommendations. A case study can focus on a business or entire industry, a specific project or program, or a person.

What are the 4 types of clinical trials? ›

Types of clinical trials
  • Pilot studies and feasibility studies.
  • Prevention trials.
  • Screening trials.
  • Treatment trials.
  • Multi-arm multi-stage (MAMS) trials.
  • Cohort studies.
  • Case control studies.
  • Cross sectional studies.
1 Feb 2022

Can case-control studies measure risk? ›

Key Concept: In a study that is designed and conducted as a case-control study, you cannot calculate incidence. Therefore, you cannot calculate risk ratio or risk difference. You can only calculate an odds ratio.

Are case-control studies prone to bias? ›

Case-control studies done in a clinical setting are even further prone to bias because the factors that bring patients to the clinical setting are often related to the disease or risk factor of interest.

What is the difference between cross sectional and case-control study? ›

Cross sectional studies are used to determine prevalence. They are relatively quick and easy but do not permit distinction between cause and effect. Case controlled studies compare groups retrospectively. They seek to identify possible predictors of outcome and are useful for studying rare diseases or outcomes.

Are case-control studies strong? ›

Although these studies are not ranked as highly as randomised controlled trials, they can provide strong evidence if designed appropriately.

Is case-control study prospective or retrospective? ›

Case-control studies are retrospective and cannot therefore be used to calculate the relative risk; this a prospective cohort study. Case-control studies can however be used to calculate odds ratios, which in turn, usually approximate to the relative risk.

What is another name for a cohort study? ›

Both types of cohort studies are also referred to as longitudinal or follow-up studies. The investigator controls the selection of the cohort. The investigator may choose a cohort based on age, location, exposure to a certain working environment, or some other common characteristic.

What is a cohort vs group? ›

Cohort can be distinguished from groups of students through the following aspects: cohorts allow school administrators to enroll students en masse while groups only concern teacher management within the class; cohorts are bigger than groups; and, a cohort involves a set of students within a system-wide course.

What are the two types of cohort study? ›

There are two types of cohort studies: Prospective and Retrospective. The two groups of cohorts (exposed and un-exposed) are followed prospectively over time to track the development of new disease.

What is the biggest disadvantage of a case study? ›

An important disadvantage is that case study research is susceptible to bias when the researcher's personal opinions and preferences interfere with the collection and analysis of data.

Is there confounding in case-control studies? ›

Confounding in case-control studies can be controlled for during the design stage. Age, locality, and time of sleep were potential confounders. Matching cases and controls on these variables ensured they were equally distributed between groups, eliminating their effects as potential confounders (d is true).

How can you avoid bias in a case-control study? ›

Rules for Avoiding Selection Bias in a Case-Control Study

Controls must be selected independently from exposure, meaning that whether or not a person is exposed or unexposed should not influence selection or enrollment of a control subject.

What is confounding in case-control study? ›

A confounding variable gives rise to situations in which the effects of two processes are not separated, or the contribution of causal factors cannot be separated, or the measure of the effect of exposure or risk is distorted because of its association with other factors influencing the outcome of the study.

What statistical analysis is used in case-control study? ›

Logistic regression modeling, in its various forms, has become by far the most frequently applied method for multivariable analysis of case-control studies.

Is a case-control study the same as a case study? ›

A case report is the description of the clinical story of a single patient, whereas a case-control study compares 2 groups of participants differing in outcome in order to determine if a suspected exposure in their past caused that difference.

Are case-control studies experimental or observational? ›

Cohort studies and case control studies are two types of observational studies.

How do you know if a study is cross-sectional or cohort? ›

Cross sectional studies are used primarily to determine the prevalence of a problem whereas cohort studies involve the study of the population that is both exposed and non-exposed to the cause of disease development agents.

What are the four types of case studies? ›

What Are The Types Of Case Study?
  • Illustrative Case Study.
  • Exploratory Case Study.
  • Cumulative Case study.
  • Critical Instance Case Study.
24 Nov 2021


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