Comunicación y Gerencia 1Case control studies15/12/2010

Case control studies2 By Associate Professor Family & Community Medicine Department King Saud University Dr. Salwa Tayel Case Control Studies 15/12/2010

Objectives 1.To understand the basic design features of case control study 2. To list the advantages and disadvantages of case control studies 3.To calculate the odds ratio as a measure of association in case control studies 3Case control studies15/12/2010

Types of Study Designs DesignStudy Type Case reportObservational - Descriptive Case seriesObservational - Descriptive Cross sectionalObservational - Descriptive Case controlObservational - Analytic CohortObservational - Analytic Clinical trialExperimental - Analytic 4Case control studies15/12/2010

Basic Question in Analytic Epidemiology Are exposure and disease linked? Direction of inquiry in case control study Exposure Risks e.g smoking Disease e.g. Myocardial Infarction (MI) 5Case control studies15/12/2010

It is an “observational” design comparing exposures in diseased cases vs. healthy controls from same population – exposure data collected retrospectively – It is the most feasible design where disease outcomes are rare Case-Control Studies 6Case control studies15/12/2010

Case Control Study Key Basis for selection of group for study = present = absent ExposureDisease ? ? 7Case control studies15/12/2010

Case-Control Design Study groups Cases (disease) Controls (no disease) factor present factor absent factor present factor absent present past time Study begins here 8Case control studies15/12/2010

Design of Case Control Study The investigator selects cases with the disease and appropriate controls without the disease (needed for comparison) -Controls must be matched with cases for certain characteristics, which are known to influence the outcome of the disease (confounding factors) e.g. age, sex, social class,….... -Controls must be matched with cases for certain characteristics, which are known to influence the outcome of the disease (confounding factors) e.g. age, sex, social class,….... 9Case control studies15/12/2010

Sources of controls: 1. General population 2. Hospital controls. 3. Special control series (Friends - neighbours- fellow employees or peers - family members)  Ratio control to cases 1 : 1 up to 4 : 1 Why we need control group? To compare the exposure rate among both case and controls (% smoking among cases and controls). 10Case control studies15/12/2010

Proportion of smoking in cases and controls controls cases Arrows show the extent of smoking among cases and controls Smoking 11Case control studies15/12/2010

Study Design Cases Controls Myocardial infarction OTHER ILLNESS/ Free of any illness EXPOSEDSMOKED NOT EXPOSED NEVER SMOKED EXPOSEDSMOKED NOT EXPOSED NEVER SMOKED Proportions of exposed and unexposed in cases and controls 12Case control studies15/12/2010

Summarize your data in 2x2 (association) table Disease Exposure YesNoTotal Yes (exposed) ab total # exposed No (unexposed) cd total # unexposed Total total # withdisease with no disease Total Population Summarize frequencies of disease and exposureSummarize frequencies of disease and exposure Calculate associationCalculate association 13Case control studies15/12/2010

Framework of a case control design ControlsCasesExposure bdbd acac Exposed Non exposed b + da +c Total Compare percent exposed among cases and controls 14Case control studies15/12/2010

Example Cases Controls Cases Controls Smokers Never smokers Total % smokers 55% 37.5% If disease is related to smoking then more cases than controls would be smokers 15Case control studies15/12/2010

Measures of association Odds ratio ~ cross product ratio CasesControls Exposed Not exposed OR= = ad/bc = [(110*250)/(150*90)] = 27500/ = 2 Odds of exposure in cases : two times compared to controls 16Case control studies15/12/2010

Interpretation of (OR) odds ratio > 1 means the exposure is a risk factor. = 1 means the exposure is not associated with the disease. < 1 means the exposure is protective 17Case control studies15/12/2010

1.Efficient, quick and cheap. 2.Useful in early stages of the development of knowledge. 3.Can be used in rare diseases. risk factors) 4.Multiple exposures (risk factors) could be studied 5.Useful in the study of disease with a long latency. 5.Useful in the study of disease with a long latency. e.g. chronic diseases 6.Does not require large samples. Advantages of case control studies: 18Case control studies15/12/2010

1.Can not calculate prevalence or incidence rates. 2. Errors in Case definitions and in characterization of exposure 3. Disease and exposure have already occurred and potential for bias remains -selection bias. -recall bias. -interviewer’s bias. Disadvantages of case control studies: 19Case control studies15/12/2010

Bias Bias is the tendency of a measurement to deviate from the true value. recall bias –occurs when cases and controls are asked about the exposure, it may be more likely for the cases to remember the existence of exposure than the controls who are healthy persons. interviewer’s bias: interviewers would keenly interview cases to obtain information compared to controls 20Case control studies15/12/2010

Case control studies21 Thank You Website 15/12/2010