1. To determine whether or not an association exists between something and a particular risk factor.
2. It may be the first step in testing a hypothesis. If positive, it can then be tested in a cohort study.
3. To start with a group of people with a certain characteristic and work back to see whether a possible risk factor may be the cause.

University of Michigan - Ann Arbor

Case-control studies are observational studies where subjects' previous experiences are used as evidence to assess how those who had experience with the specified condition varied from those who did not and why.

Case-control studies

James, G., Witten, D., Hastie, T., & Tibshirani, R. (2013). An introduction to statistical learning (Vol. 112, pp. 3-7). New York: springer.

An Introduction to Statistical Learning with Applications in R

1. Define a study population (source of cases and controls) 
2. Define and select cases.
3. Define and select controls.
4. Measure exposure.

Four steps in conducting a case-control study

Purpose of case control studies

A case control study is an observational study method that selects people with a disease and those without a disease to investigate their exposure to a risk factor and its degree respectively, so as to determine whether the exposure risk factor is related to a disease and the degree of association.

Drawbacks: (Tobacco case as an example)
-  Retrospective: We know the subject has cancer, and in that context we have to go back and figure out why
-  Reversed logic of probability: What these data tell us is the probability that cancer patients are smokers, not the probability that smokers get cancer. For those who want to know if they should smoke, the probability of cancer in smokers is the probability they really care about
-  Recall bias: While the interviewers may be blinded to cancer vs. control, the patients themselves know whether or not they had cancer. Having already gone through this event can affect their recollections
-  Selection bias: Hospitalized cancer patients (aka those on record as having cancer and those specifically hospitalized for it) are not necessarily representative of the population at large or the smoking population at large.

Advantages of RCTs over Case Control Studies

Advantages of Case Control Studies:
- Quicker, cheaper and require less time and effort than cohort studies
- Case-control studies can study multiple risk factors/exposures
- They are useful for studying outcomes that take a long time to develop

Drawbacks of RCTs: 
- Ethics: RCT approaches are usually controversial and must be handled with care. The experimental group (intervened group) and control group (not- intervened group) are likely to have different outcomes and impact people who get involved in the trial. For example, you cannot assign people to smoke for decades just to figure out if they would get cancer. 
- Feasibility: Running RCTs can be very expensive. They cost time, effort and money. It can take decades to assess the long-term effect of a vaccine. Due to the high costs, most RCTS only inspect one or very few variables. Therefore they rarely reflect the full picture of a complicated issue.


Advantages of Case Control Studies over RCTs


An observational study method that selects people with a disease and those without a disease to investigate their exposure to a risk factor and its degree respectively, so as to determine whether the exposure risk factor is related to a disease and the degree of association.
Drawbacks: (Tobacco case as an example)
- Retrospective: We know the subject has cancer, and in that context we have to go back and figure out why.
- Reversed logic of probability: What these data tell us is the probability that cancer patients are smokers, not the probability that smokers get cancer. For those who want to know if they should smoke, the probability of cancer in smokers is the probability they really care about.
- Recall bias: While the interviewers may be blinded to cancer vs. control, the patients themselves know whether or not they had cancer. Having already gone through this event can affect their recollections.
- Selection bias: Hospitalized cancer patients (aka those on record as having cancer and those specifically hospitalized for it) are not necessary representative of the population at large or the smoking population at large. 

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