Case-control
Encyclopedia
A case-control study is a type of study design
in epidemiology
. Case-control studies are used to identify factors that may contribute to a medical condition by comparing subjects who have that condition (the 'cases') with patients who do not have the condition but are otherwise similar (the 'controls').
Case-control studies are a relatively inexpensive and frequently-used type of epidemiological study that can be carried out by small teams or individual researchers in single facilities in a way that more structured experimental studies often cannot be. They have pointed the way to a number of important discoveries and advances.
The great triumph of the case-control study was the demonstration of the link between tobacco smoking
and lung cancer
, by Sir Richard Doll
and others after him. Doll was able to show a statistically significant association between the two in a large case control study. Opponents argued for many years that this type of study cannot prove causation, but the eventual results of cohort studies
confirmed the causal link which the case-control studies suggested, and it is now accepted that tobacco smoking
is a cause of about 87% of all lung cancer
mortality in the US.
. 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.
Porta's Dictionary of Epidemiology defines the case-control study as an "observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease (comparison group, reference group). The potential relationship of a suspected risk factor or an attribute to the disease is examined by comparing the diseased and nondiseased subjects with regard to how frequently the factor or attribute is present (or, if quantitative, the levels of the attribute)in each of the groups (diseased and nondiseased)." The case-control study is frequently contrasted with cohort studies, wherein exposed and unexposed subjects are observed until they develop an outcome of interest.
Case-control studies are observational in nature and thus do not provide the same level of evidence as randomized controlled trials. It may also be more difficult to establish the timeline of exposure to disease outcome in the setting of a case-control study than within a prospective cohort study design where the exposure is ascertained prior to following the subjects over time in order to ascertain their outcome status. The most important drawback in case-control studies relates to the difficulty of obtaining reliable information about an individual’s exposure status over time. It should however be noted that many high quality and reliable case-control studies have been carried out and have produced useful results.
and lung cancer
, by Sir Richard Doll
and others after him. Doll was able to show a statistically significant association between the two in a large case-control study. Opponents argued for many years that this type of study cannot prove causation, but the eventual results of cohort studies
confirmed the causal link which the case-control studies suggested, and it is now accepted that tobacco smoking
is the cause of about 87% of all lung cancer
mortality in the US.
Study design
Clinical study design is the formulation of trials and experiments in medical and epidemiological research, sometimes known as clinical trials. Many of the considerations here are shared under the more general topic of design of experiments but there can be others, in particular related to patient...
in epidemiology
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...
. Case-control studies are used to identify factors that may contribute to a medical condition by comparing subjects who have that condition (the 'cases') with patients who do not have the condition but are otherwise similar (the 'controls').
Case-control studies are a relatively inexpensive and frequently-used type of epidemiological study that can be carried out by small teams or individual researchers in single facilities in a way that more structured experimental studies often cannot be. They have pointed the way to a number of important discoveries and advances.
The great triumph of the case-control study was the demonstration of the link between tobacco smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...
and lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
, by Sir Richard Doll
Richard Doll
Sir William Richard Shaboe Doll CH OBE FRS was a British physiologist who became the foremost epidemiologist of the 20th century, turning the subject into a rigorous science. He was a pioneer in research linking smoking to health problems...
and others after him. Doll was able to show a statistically significant association between the two in a large case control study. Opponents argued for many years that this type of study cannot prove causation, but the eventual results of cohort studies
Cohort study
A cohort study or panel study is a form of longitudinal study used in medicine, social science, actuarial science, and ecology. It is an analysis of risk factors and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction...
confirmed the causal link which the case-control studies suggested, and it is now accepted that tobacco smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...
is a cause of about 87% of all lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
mortality in the US.
Definition
The case-control is a type of epidemiological observational studyObservational study
In epidemiology and statistics, an observational study draws inferences about the possible effect of a treatment on subjects, where the assignment of subjects into a treated group versus a control group is outside the control of the investigator...
. 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.
Porta's Dictionary of Epidemiology defines the case-control study as an "observational epidemiological study of persons with the disease (or another outcome variable) of interest and a suitable control group of persons without the disease (comparison group, reference group). The potential relationship of a suspected risk factor or an attribute to the disease is examined by comparing the diseased and nondiseased subjects with regard to how frequently the factor or attribute is present (or, if quantitative, the levels of the attribute)in each of the groups (diseased and nondiseased)." The case-control study is frequently contrasted with cohort studies, wherein exposed and unexposed subjects are observed until they develop an outcome of interest.
Usefulness and drawbacks
One useful trait of case-control studies is that they tend to be less costly to carry out than prospective cohort studies, as well as having the potential to be shorter in duration. Another aspect is the greater statistical power of this type of study in several situations, given the fact that cohort studies must often wait for a 'sufficient' number of disease events to accrue.Case-control studies are observational in nature and thus do not provide the same level of evidence as randomized controlled trials. It may also be more difficult to establish the timeline of exposure to disease outcome in the setting of a case-control study than within a prospective cohort study design where the exposure is ascertained prior to following the subjects over time in order to ascertain their outcome status. The most important drawback in case-control studies relates to the difficulty of obtaining reliable information about an individual’s exposure status over time. It should however be noted that many high quality and reliable case-control studies have been carried out and have produced useful results.
Example of a case-control study
One of the most significant triumphs of the case-control study was the demonstration of the link between tobacco smokingTobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...
and lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
, by Sir Richard Doll
Richard Doll
Sir William Richard Shaboe Doll CH OBE FRS was a British physiologist who became the foremost epidemiologist of the 20th century, turning the subject into a rigorous science. He was a pioneer in research linking smoking to health problems...
and others after him. Doll was able to show a statistically significant association between the two in a large case-control study. Opponents argued for many years that this type of study cannot prove causation, but the eventual results of cohort studies
Cohort study
A cohort study or panel study is a form of longitudinal study used in medicine, social science, actuarial science, and ecology. It is an analysis of risk factors and follows a group of people who do not have the disease, and uses correlations to determine the absolute risk of subject contraction...
confirmed the causal link which the case-control studies suggested, and it is now accepted that tobacco smoking
Tobacco smoking
Tobacco smoking is the practice where tobacco is burned and the resulting smoke is inhaled. The practice may have begun as early as 5000–3000 BCE. Tobacco was introduced to Eurasia in the late 16th century where it followed common trade routes...
is the cause of about 87% of all lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...
mortality in the US.
Analysis of case-control studies
Case-control studies were initially analyzed by testing whether or not there were significant differences between the proportion of exposed subjects among cases and controls. Subsequently Cornfield pointed out that, when the disease outcome of interest is rare, the odds ratio of exposure can be used to estimate the relative risk (see rare disease assumption). It was later shown by Miettinen in 1976 that this assumption is not necessary and that the odds ratio of exposure can be used to directly estimate the incidence rate ratio of exposure without the need for the rare disease assumption.See also
- Nested case-control studyNested case-control studyA nested case control study is a variation of a case-cohort study in which only a subset of controls from the cohort are compared to the incident cases. In a case-cohort study, all incident cases in the cohort are compared to a random subset of participants who do not develop the disease of interest...
- Epi InfoEpi InfoEpi Info is public domain statistical software for epidemiology developed by Centers for Disease Control and Prevention in Atlanta, Georgia ....
software program - OpenEpiOpenEpiOpenEpi is a free, web-based, open source, operating system-independent series of programs for use in epidemiology, biostatistics, public health, and medicine, providing a number of epidemiologic and statistical tools for summary data. OpenEpi was developed in JavaScript and HTML, and can be run in...
software program
Further reading
(Still a very useful book, and a great place to start, but now a bit out of date.)External links
- A list of computer programs for genetic analysis including case-control genetic association analysis
- Study Design Tutorial Cornell University College of Veterinary Medicine
- Wellcome Trust Case Control Consortium