Present: Disease Past: Exposure CHP400: Community Health Program- lI STUDY DESIGNS Observational / Analytical Studies Time is Key Research Methodology nbmmb Present: Disease & Exposure Present: Exposure Future: Disease Cross - section Cohort Case Control Studies Case - control  Present: Disease Past: Exposure

Content Definition and importance of Case - Control Studies Design Selecting Cases Selecting Controls Assessing Exposure: Odds Ratio (OR) Advantages Disadvantages

Case Control Studies Research Methodology STUDY DESIGNS Experimental Observational Experimental Animal Experiment Descriptive Analytical Case Control Studies Case report Case control Case series Human Intervention Clinical trial Cross section Cohort Ecological

Observational / Analytical Studies Case - Control Studies Definition: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Definition: It is a type of observational analytic epidemiologic investigation in which subjects are selected on the basis of whether they do or do not have the particular disease under study.

Case - Control Studies Importance: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Importance: The most frequently undertaken analytical epidemiological studies The only practical approach for identifying risk factors for rare diseases They are best suited to the study of diseases for which medical care is sought, such as cancers or hip fracture

Case - Control Studies Design : At baseline: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Design : At baseline: Selection of cases (disease) and controls (no disease) based on disease status Exposure status is unknown Retrospective design – lacks temporality!

Case - Control Studies Design : Exposed Diseased Not Exposed Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Design : Exposed Diseased (Cases) Not Exposed Target Population Exposed Not Diseased (Controls) Not Exposed

Case - Control Studies Design :  ? YES ? NO Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Design :  EXPOSURE OUTCOME/DISEASE TIME ? YES ? NO

Case - Control Studies Selecting Cases: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Selecting Cases: Select cases after the diagnostic criteria and definition of the disease is clearly established Selected cases should be representative of all cases

Case - Control Studies Selecting Cases: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Selecting Cases: The study need not include all cases in the population Cases may be selected from hospitals, clinics, disease registries, screenings, etc.

Case - Control Studies Selecting Cases: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Selecting Cases: Incident cases are preferable to prevalent cases for reducing (a) recall bias and (b) over-representation of cases of long duration The most desirable way to obtain cases is to include all incident cases in a defined population over a specified period of time

Case - Control Studies Selecting Controls: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Selecting Controls: Controls should come from the same population at risk for the disease as the cases Controls should be representative of the target population

Case - Control Studies Selecting Controls: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Selecting Controls: Multiple controls can be used to add statistical power when cases are difficult to obtain Using more than one control group lends credibility to the results More than 3 controls for a case is usually not cost-efficient

Sources of cases and controls Selecting Controls: Sources of cases and controls CASES CONTROLS All cases diagnosed in the community Sample of general population Non-cases in a sample of the population All cases diagnosed in a sample of the population All cases diagnosed in all hospitals Sample of patients in all hospitals who do not have the disease All cases diagnosed in a single hospital Sample of patients in the same hospital who do not have the disease Any of the above methods Spouses, siblings or associates of cases

Case - Control Studies Analysis: Odds Ratio (OR) Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Analysis: Odds Ratio (OR) A ratio that measures the odds of exposure for cases compared to controls Odds of exposure = number exposed  number unexposed OR Numerator: Odds of exposure for cases OR Denominator: Odds of exposure for controls Odds: the possibility that something will happen : the chance that one thing will happen instead of a different thing An odds ratio (OR) is a measure of association between an exposure and an outcome. The OR represents the odds that an outcome will occur given a particular exposure, compared to the odds of the outcome occurring in the absence of that exposure. Odds ratios are most commonly used in case-control studies, however they can also be used in cross-sectional and cohort study designs as well (with some modifications and/or assumptions

Case - Control Studies Analysis: Odds Ratio (OR) Disease Status = = = Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Analysis: Odds Ratio (OR) Disease Status CHD cases (Cases) No CHD (Controls) Smoker 112 176 Exposure Status a b Non-smoker 88 c d 224 Total 200 400 a b ad 112 x 224 Odds Ratio = = = = 1.62 c d bc 176 x 88

Interpreting the Odds Ratio (e.g) OR<1 OR=1 OR>1 Odds of exposure are equal among cases and controls Odds of exposure for cases are greater than the odds of exposure for controls Odds comparison between cases and controls Odds of exposure for cases are less than the odds of exposure for controls Exposure is not a risk factor nor a protective factor Exposure as a risk factor for the disease? Exposure increases disease risk (Risk factor) Exposure reduces disease risk (Protective factor)

Interpreting the Odds Ratio Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Analysis: Odds Ratio (OR) Interpreting the Odds Ratio The odds of exposure for cases are 1.62 times the odds of exposure for controls. Those with CHD are 1.62 times more likely to be smokers than those without CHD

Possible Sources of Bias and Error Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Possible Sources of Bias and Error Information on the potential risk factor (exposure) may not be available: - either from records - or the study subjects’ memories

Case - Control Studies cont. Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies cont. Cases may search for a cause for their disease and thereby be more likely to report an exposure than controls (recall bias) The investigator may be unable to determine with certainty whether the suspected agent caused the disease or whether the occurrence of the disease caused the person to be exposed to the agent

Case - Control Studies cont. Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies cont. Identifying and assembling a case group representative of all cases may be unduly difficult Identifying and assembling an appropriate control group may be unduly difficult

Several exposures can be studied. Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Advantages: Quick and easy to complete, cost effective Most efficient design for rare diseases Usually requires a smaller study population than a cohort study Several exposures can be studied.

Case - Control Studies Disadvantages: Research Methodology STUDY DESIGNS Observational / Analytical Studies Case - Control Studies Disadvantages: Uncertainty of exposure-disease time relationship Inability to provide a direct estimate of risk Not suitable for studying rare exposures Subject to biases (recall & selection bias)

Summary Case-control study: Observational, analytic study. Most frequently undertaken analytical studies Quick and easy to complete, cost effective Most efficient design for rare diseases Subjects are selected on the basis of presence or absence disease under study Odds Ratio (OR)