Case Control Studies are typically observational studies commonly used to outline factors related with certain diseases or outcomes. Selection of participants is done on the basis of an experienced outcome. However, to introduce the control aspect within the study, other participants are selected at random from the population without having experienced that outcome. In both the cases and controls participants, exposure is assessed retrospectively through medical records and interviews.
Hierarchy of Evidence
Case Control Studies Participant Selection Criteria
CASES
- clear inclusion/exclusion criteria to ensure homogeneity
- cases should ideally be representative of the cases within the target population for external validity purposes
- cases should be sourced from the community, clinic, or hospital
- accurate diagnosis is important so as not to dilute the cases group with those who do not actually have the disease in question
CONTROLS
- controls should be selected from the same population, and may include individuals at risk of developing the outcome
- same inclusion/exclusion criteria but without the outcome should be used, with the emphasis being on comparability of cases and controls
- accurate classification of controls should be ensured; if confounders are known, they should be matched through a matched study, otherwise, confounders need to be considered in data analysis, and a bigger sample would be required
Matching
Matching is an attempt to ensure comparability between the cases and controls. Matching reduces variability and systematic differences caused by extraneous variables a.k.a. confounders (such as age, gender and race), which may be related to the risk factor.
Bias
INTERVIEWER BIAS – interviewer asks the leading questions, which are different from those used for the control group.
DATA QUALITY – incomplete or inaccurate data
RECALL BIAS – Participants with the disease (CASES) are more likely to recall and report exposure due to having experienced the outcome
Advantages VS Disadvantages
| ADVANTAGES | DISADVANTAGES |
| ideal when seeking possible causes of rare outcomes and outcomes with long latency | may be difficult to select appropriate controls group |
| does not require a large group of participants | extraneous variables a.k.a. confounder control may be incomplete |
| relatively quick since the outcome would have already occurred | difficult to validate information |
| multiple exposures or risk factors can be examined | susceptible to recall bias |
| relatively inexpensive |
Performing Case-Control Studies
- cases are identified
- control group individuals with similar characteristics but without the outcome in question are identified
- exposure is measured retrospectively in both groups
- occurrence rate of exposure in cases is compared to the occurrence rate of exposure in control
- results are typically obtained through odds ratios or relative risk: show occurrence in exposed is divided by occurrence in non-exposed; if value is 1 = no difference; if value is >1 = risk is higher in exposed; if value is <1 = risk is higher in non=exposed
Cohort Study VS Case Control Study
CASP Tool for Case Control Studies
CASP Tool for case-control studies can be accessed here.
To view blogpost featuring Cochrane videos on all types of studies please click here.
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