Case Control study

An investigation that compares a group of people with a disease to a group of people without the disease. Used to identify and assess factors that are associated wit diseases or health conditions, More common than the cohort study. Enroll a group of people with disease (“cases”) and a group without disease (“controls”) and compare their patterns of previous exposures. The key in a case-control study is to identify an appropriate control, or comparison group, because it provides our measure of the expected amount of exposure. Not need to know the size of the total exposed group and the total unexposed group true size of the population from which the cases came is usually not known, so we usually cannot calculate rates or a relative risk. Can still calculate an odds ratio, and interpret it as an approximation of the relative risk. Information about exposure generally collected after the disease occurred, Then sometimes called retrospective studies. Starts with outcome or disease instead of exposure in cohort studies Case Control study characters

Schematic diagram of a case-control study.

Identification of cases Any case could be enrolled,

Identification of controls One of the most difficult design issues, differences may be because of an error: selection bias, Controls should be representative of the source population from which cases derived, Best way is the random sample from the source population which the cases came, Other sources are the neighbours

Analysis of data After identification of interest (disease or health condition) and the factors to be studied, A method for collection of information: Questionnaire, clinical examination, telephone call and … Questions should include information about the outcome of interest and the factors under study, Analysis includes calculating the odds ratio as a measure of association between the disease and each of the factors of interest, (any association and its magnitude)

Definition of Odds Ratio Sometimes called relative odds, A measure of association between exposure and the disease or outcome, Used in case control studies, in which incidence rates cannot be calculated, Contrasts the odds of exposure among cases with the odds of exposure among controls,

Case-control studies are an efficient method for studying rare diseases. Quick to run considerably cheaper than other study types. useful for studying diseases with long incubation or latency period, Advantages

No incidence rate, sometimes mistakes chronologic order sometimes selection bias, information bias: recall bias good for rare diseases but not rare exposure not suitable for studying multiple diseases or conditions because of the controls Disadvantages

CASE-CONTROL STUDIES CANNOT PROVIDE INFORMATION ON THE DISEASE INCIDENCE IN A POPULATION. IS RELIANT ON THE QUALITY OF PAST RECORDS OR RECOLLECTION OF STUDY PARTICIPANTS. Disadvantages

جدول اتفاقات Contingency table (Typical 2*2 Table for a case-control study) بیمار ( مورد ) سالم ( كنترل ) دارای تماس a b بدون تماس c d جمع a+c b+d

توصيف جدول اتفاقات

Odds of exposure among the cases = a/c Odds of exposure among the controls = b/d We calculate the odds in favor of exposure not disease which we interest, Measure of association in case – control studies

Odds ratio interpretation OR = 1 suggests No association, OD>1 suggests a possible risk factor, OD>1 suggests a possible protective factor,

Perhaps most important, decision is who the controls should be they should be similar to the cases except that they don’t have the disease. The controls would provide us with the level of exposure that you should expect to find among the cases in an outbreak of 50 or more cases, 1 control per case will usually suffice. In smaller outbreaks, you might use 2, 3, or 4 controls per case. More than 4 controls per case will rarely be worth your effort. If exposure is much higher among the cases than the controls, you might choose to reject the null hypothesis in favor of a hypothesis that says exposure is associated with disease. Biases in the control group can distort the data in either direction, نکات مهم جهت رعايت

Muscat et al (2000) sought to test the hypothesis that cellular telephone use affects the risk of brain cancer. From 1994 to 1998 at five academic medical centres in the USA they recruited 469 cases aged 18 to 80 years with newly diagnosed cancer originating in the brain. Controls (n = 422) were inpatients without brain cancer at those hospitals, excluding those with leukaemia or lymphoma. Controls were sampled to match the cases on age, sex, race and month of admission. Each case and control was then interviewed about any past subscription to a cellular telephone service. Overall 14.1% of cases and 18.0% of controls reported ever having had a subscription for a cellular telephone service. After adjusting for age, sex, race, education, study centre, and month and year of interview, the risk of developing brain cancer in a cellular telephone user was estimated to be 0.85 (95% ci 0.6 – 1.2) times as great as in a non-user. An example of case – control study