Epidemiology MPH 531 Analytic Epidemiology Case control studies Mpundu MKC MSc Epidemiology and Biostatistics, BSc Nursing, RM, RN

Case control studies Definition An analytic epidemiologic study design in which the study population consist of groups, one with the disease/condition and a group without the disease (controls) and compare the patterns of the exposure (risk factor)

Case-Control Studies Definition: An observational analytic epidemiologic study which examines the association between exposure and a health outcome by comparing cases or individuals who develop outcome and controls who are a sample of the source population from which the cases were identified

Case-Control Studies Two groups are selected, one of people with the disease (cases), and the other of people with the same general characteristics but without the disease (controls) Compare the past exposures of both groups to determine if the exposure could account for the health condition in case

Case-Control studies Direction Subjects selected on the basis of Disease Start with persons having Disease Compare frequency of Exposure in cases (Disease) with frequency of Exposure in controls (non-Disease) Look backward for history of Exposure Direction always backward Exposure Disease

Case-Control Studies Timing Review timing: Disease happened when study begin Basic design: Retrospective Disease as occurred before onset of study

Characteristics of Case-Controls Observational study Explanatory (Analytical) Retrospective Disease to Exposure Both Exposure and Disease have already occurred Uses comparison group

Case-Control Studies Disease ( 𝑫 + ) Not Exposed Source Population Exposed Controls ( 𝐷 − )

Possible Sources of Cases Hospitals, Acute and emergency (Casualty) Clinics, physician offices Laboratories Disease registries Surveillance systems Vital records (Central Statistical office, Council) General population

Case-Control Studies Selection of cases Case definition Inclusion and exclusion criteria

e.g. of CASES Presence of disease, injury, disability or death Delayed immunisation Low birth weight Drug Resistance (MDR-TB)

Case-Control Studies Control Selection Controls should Represent source population from which the cases were derived Represent persons who, if a case, would have been in the study Be selected independently of Exposure

Odds Ratio Calculation Outcome Exposure Cases Controls Exposed A B C D Not Exposed estimates the relative risk Odds of exposure for cases A / C =Odds Ratio Odds of exposure for controls B / D

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

Comparing Odds Ratios and Relative Risks Outcome Exposure Cases Controls Exposed 70 300 370 30 700 730 Not Exposed 1100 100 1000

e.g. calculations Relative Risk = 4.41 Odds Ratio = AD/BC = 5.44

Calculating the Odds Ratio Disease Status CHD cases (Cases) No CHD (Control) Exposure Status Smokers 132 176 88 324 Non-smokers Total 220 500

e.g of calculations Odds(smokers) =case/control=132/176 Odds(nosmokers) =case/control=88/234

Odds Ratio = AD BC = 132 x 324 176 x 88 OR= 2.76

Case-control Studies Measures of Association Odds Ratio also known as Ratio of Two Odds Estimator of relative risk Odds of Exposed among cases compared with Exposure among controls Also known as exposure odds ratio - OR = 1 – no association - OR > 1 – Exposure is a risk factor - OR < 1 – Exposure is protective

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 as a risk factor for the disease? Particular exposure is not a risk factor Exposure increases disease risk (Risk factor) Exposure reduces disease risk (Protective factor)

Case-control Studies Common Biases Selection bias Information bias - recall bias Confounding

Case Control Studies Advantages Quicker, cheaper and require less time and effort than cohort studies Case-control studies can study rare diseases Case-control studies can study multiple risk factors/exposures They are useful for studying outcomes (diseases) that take a long time to develop, e.g. cancer

Case CONTROL STUDIES Disadvantages Case-control studies are prone to selection and recall bias (i.e. better recollection of exposure amongst cases than among controls They are inefficient for examining rare exposures It may be difficult to establish temporality (when the person was actually exposed to the disease/risk factor)

Case CONTROL STUDIES Disadvantages It can be difficult to choose an appropriate control (comparison) group Case-control studies cannot calculate incidence rates, relative risks or attributable risks. Instead odds ratio are the measure of association used (when outcome is uncommon, e.g. most cancers, it can be a good proxy for the true relative risk Incomplete data

Summary Case-control Studies One of the major types of analytic epidemiological study designs Start with disease and look backward for exposure Odds ratio is standard measure of association