# Epidemiology: An Introduction

Pioneers in Epidemiology and Public HealthORIGINS OF PUBLIC HEALTHFROM HIPPOCRATES TO SNOW AND THE MODERN DAYHippocrates (~460-370 bc)Avicenna (Ibn Sina) (980-1037)Fracastoro (1478-1553)John Graunt (1620-1674)Bernardino Ramazzini (1633-1714)William Farr (1807-1883)John Snow (1813-1858)Ignasz Semmelweis (1818-1865)Florence Nightingale (1820-1910)Janet Lane-Claypon (1877-1967)Wade Hampton Frost (1880-1938)ALSO NOTEWORTHYQUESTIONSADDITIONAL READINGREFERENCESWhat Is Causation?THE CAUSAL PIE MODELImplications of the Causal Pie ModelGenetic Versus Environmental CausesStrength of CausesInteraction Between CausesSum of AttRiBu table FractionsInduction TimeIs a Catalyst a Cause?THE PROCESS OF SCIENTIFIC INFERENCECausal CriteriaGeneralization in EpidemiologyQUESTIONSREFERENCESMeasuring Disease Occurrence and Causal EffectsMEASURES OF DISEASE OCCURRENCERisk and Incidence ProportionAttACK Rate and Case-Fatality RateIncidence RateChicken and EggThe Relation Between Risk and Incidence RatePoint-Source and Propagated EpidemicsPrevalence ProportionPrevalence of CharacteristicsMEASURES OF CAUSAL EFFECTSThe Counterfactual IdealEFFECT MEASURESWhen to Use Absolute and Relative Effect MeasuresExamplesThe Relation Between Risk Ratios and Rate RatiosWhen Risk Does Not Mean RiskAttributable FractionQUESTIONSREFERENCESTypes of Epidemiologic StudiesCOHORT STUDIESJohn Snow’s Natural ExperimentTypes of ExperimentsNatural Experiments Are Not ExperimentsPopulation at RiskExperiments Are an Imperfect Gold StandardCohort Study of Vitamin A During Pregnancy: An ExampleDisease-Free Does Not Imply HealthyClosed and Open CohortsCounting Disease EventsMeasuring Incidence Rates or RisksCohort Study of X-Ray Fluoroscopy and Breast Cancer: An ExampleExposure and Induction TimeEligibility Criteria, Exposure Classification, and Time LoopsWhich Measures to Report from Cohort Studies?Retrospective Cohort StudiesTracing of SubjectsSpecial Exposure and General Population CohortsCASE-CONTROL STUDIESNested Case-Control StudiesBasic Types of Case-Control StudiesDensity Case-Control StudiesDefining the Source PopulationControl SelectionIllustration of Density-Based Case-Control DataCumulative Case-Control StudiesCase-Cohort StudiesIllustration of Case-Cohort DataSources for Control SeriesProspective and Retrospective Case-Control StudiesIs Representativeness Important?Case-Crossover StudiesCROSS-SECTIONAL VERSUS LONGITUDINAL STUDIESRESPONSE RATESCOMPARISON OF COHORT AND CASE-CONTROL STUDIESQUESTIONSREFERENCESInfectious Disease EpidemiologyTYPES OF TRANSMISSIONHERD IMMUNITY AND BASIC REPRODUCTIVE NUMBERTHE REED-FROST EPIDEMIC MODELINFECTIOUS DISEASE EPIDEMIOLOGY INVESTIGATIONSOUTLOOK FOR INFECTIOUS DISEASE EPIDEMIOLOGYQUESTIONSREFERENCESSUGGESTED READINGDealing with BiasesSOURCES OF BIAS IN EPIDEMIOLOGIC STUDIESSelection BiasMatching in Case-Control StudiesInformation BiasConfoundingConfounding by IndicationProperties of a Confounding FactorControl of ConfoundingIs Confounding in a Randomized Experiment a Bias?QUESTIONSREFERENCESRandom Error and the Role of StatisticsESTIMATIONPOINT ESTIMATES, CONFIDENCE INTERVALS, AND P VALUESSTATISTICAL HYPOTHESIS TESTING VERSUS ESTIMATIONP-VALUE (CONFIDENCE INTERVAL) FUNCTIONSSIMPLE APPROACHES TO CALCULATING CONFIDENCE INTERVALSQUESTIONSREFERENCESAnalyzing Simple Epidemiologic DataCONFIDENCE INTERVALS FOR MEASURES OF DISEASE FREQUENCYRisk Data and Prevalence DataExample: Confidence Limits for a Risk or PrevalenceIncidence Rate DataExample: Confidence Limits for an Incidence RateCONFIDENCE INTERVALS FOR MEASURES OF EFFECTCohort Studies with Risk Data or Prevalence DataExposed UnexposedExample: Confidence Limits for Risk Difference and Risk RatioConfidence Intervals Versus Confidence LimitsCohort Studies with Incidence Rate DataExample: Confidence Limits for Incidence Rate Difference and Incidence Rate RatioCase-Control StudiesCALCULATION OF P VALUESRisk DataIncidence Rate DataCase-Control DataQUESTIONSREFERENCESControlling Confounding by Stratifying DataAN EXAMPLE OF CONFOUNDINGUNCONFOUNDED EFFECT ESTIMATES AND CONFIDENCE INTERVALS FROM STRATIFIED DATACohort Studies with Risk Data or Prevalence DataResidual ConfoundingConfidence Intervals for Pooled EstimatesCohort Studies with Incidence Rate DataCase-Control StudiesSTANDARDIZATIONCrude Rates and Standardized RatesWhat Is an SMR?CALCULATION OF P VALUES FOR STRATIFIED DATAMEASURING CONFOUNDINGSTRATIFICATION BY TWO OR MORE VARIABLESSTRATIFICATION AFTER MATCHINGIMPORTANCE OF STRATIFICATIONQUESTIONSREFERENCESMeasuring InteractionsEFFECT-MEASURE MODIFICATIONEffect Modification Versus Effect-Measure ModificationPooling and a Multiplicative RelationA DEFINITION OF BIOLOGIC INTERACTIONPARTITIONING THE RISK AMONG THOSE WITH JOINT EXPOSUREAssessing Biologic Interaction with Preventive FactorsIndependence Is Not a ModelQUESTIONSREFERENCESUsing Regression Models in Epidemiologic AnalysisTHE GENERAL LINEAR MODELTRANSFORMING THE GENERAL LINEAR MODELTHE LOGISTIC TRANSFORMATIONCHOICES AMONG MODELSCONTROL OF CONFOUNDING WITH REGRESSION MODELSPREDICTING RISK FOR A PERSONSTRATEGY FOR CONSTRUCTING REGRESSION MODELS FOR EPIDEMIOLOGIC ANALYSISCentering of Variables in Regression ModelsDo a Stratified Analysis FirstDetermine Which Confounders to Include in the ModelStepwise Models in Epidemiologic AnalysisEstimate the Shape of the Exposure-Disease RelationEvaluate InteractionOVERFITTING OF REGRESSION MODELS AND SUMMARY CONFOUNDER SCORESEXAMPLE OF USING PROPENSITY SCORES: ARE DRUGELUTING STENTS BETTER THAN BARE-METAL STENTS?Variable Matching Ratios, Confounding, and TrimmingSUMMARYQUESTIONSREFERENCESEpidemiology in Clinical SettingsDIAGNOSISThe Gold StandardSensitivity and SpecificityPredictive ValueScreeningLead-Time BiasPrognostic Selection BiasLength-Biased SamplingPROGNOSISTHERAPYClinical TrialsBlinding and Use of PlacebosEthics of Placebo Use in Randomized TrialsThreats to Validity in TrialsIncomplete Follow-upIntent-to-Treat AnalysisConfounding ImbalancesAn Unrejectable Null HypothesisPharmacoepidemiologyWhen the Disease Definition Includes an ExposureHEALTH OUTCOMES RESEARCHQUESTIONSREFERENCES