Therearemanygoodepidemiologytextbooksonthemarket,butmostoftheseare addressedtostudentsofpublichealthorpeoplewhodoclinicalresearchwithe- demiologicmethods. Thereisaneedforashortintroductiononhowepidemiologic methodsareusedinpublichealth,geneticandclinicalepidemiology,becausehealth professionalsneedtoknowbasicepidemiologicmethodscoveringetiologicaswell asprognosticfactorsofdiseases. Theyneedtoknowmoreaboutmethodologythan introductorytextsonpublichealthhavetooffer. Insomehealthfaculties,epidemiologyisnotevenpartoftheteachingcurri- lum. Webelievethistobeaseriousmistake. Medicalstudentsarestudentsofall aspectsofdiseasesandhealth. Withoutknowingsomethingaboutepidemiologythe cliniciansandotherhealthprofessionalscannotreadagrowingpartofthesci- ti cliteratureinanyreasonablycriticalwayandcannotnavigateintheworldof "evidence-basedmedicineandevidence-basedprevention. "Withoutskillsine- demiologicmethodologytheyareinthehandsofexpertsthatmaynotonlyhavean interestinhealth. Some health professionals may believe that only common sense is needed to conductepidemiologicalstudies,butthescienti cliteratureandthepublicdebate onhealthissuesindicatethatcommonsenseisofteninshortsupplyandmaynot thrivewithoutsomeformaltraining. Epidemiologic methods play a key role in identifying environmental, social, and genetic determinants of diseases. Clinical epidemiology addresses the tr- sitionfromdiseasetohealthortowardmortalityorsocialormedicalhandicaps. Publichealthepidemiologyaddressesthetransitionfrombeinghealthytobeingnot healthy. Descriptiveepidemiologyprovidesthediseasepatternthatisneededtolook athealthinabroadperspectiveandtosettheprioritiesright. Epidemiologyisabasic scienceofmedicinewhichaddresseskeyquestionssuchas"Whobecomesill?"and "Whatareimportantprognosticfactors?"Answerstosuchquestionsprovidethe basisforbetterpreventionandtreatmentofdiseases. Many people contributed to the writing of this book: medical students in Denmark,studentsofepidemiologyattheIEAEEPEsummercourseinFlorence, Italy,andstudentsofpublichealthinLosAngeles. Withouttechnicalassistance v vi Preface fromGitteNielsen,JenadeShelley,NinaHoheandPamMasangkaythebookwould neverhavematerialized. LosAngeles,California JornOlsen Odense,Denmark KaareChristensen IowaCity,Iowa JeffMurray Stockholm,Sweden AndersEkbom Contents Part I Descriptive Epidemiology 1 Measures of Disease Occurrence ...3 IncidenceandPrevalence ...4 Incidence...6 RatesandDynamicPopulations ...7 CalculatingObservationTime...9 Prevalence,Incidence,Duration ...10 MortalityandLifeExpectancy ...11 LifeExpectancy ...12 References...13 2 Estimates of Associations ...15 3 Age Standardization...19 4 Causes of Diseases ...23 References...28 5 Descriptive Epidemiology in Public Health...29 GraphicalModelsofCausalLinks ...33 References...35 6 Descriptive Epidemiology in Genetic Epidemiology...37 OccurrenceDatainGeneticEpidemiology ...37 ClusteringofTraitsandDiseasesinFamilies ...38 TheOccurrenceofGeneticDiseases ...40 References...41 7 Descriptive Epidemiology in Clinical Epidemiology...43 SuddenInfantDeathSyndrome(SIDS)...44 CytologicalScreeningforCervixCancer ...45 ChangesinTreatmentofJuvenileDiabetes ...46 References...47 vii viii Contents Part II Analytical Epidemiology 8 Design Options...51 CommonDesignsUsedtoEstimateAssociations...51 EcologicalStudy ...52 Case-ControlStudy...54 CohortStudy ...5 5 ExperimentalStudy ...56 Reference ...57 9 Follow-Up Studies ...59 TheNon-experimentalFollow-Up(Cohort)Study ...59 StudyingRiskasaFunctionofBMI ...60 LongitudinalExposureData...62 DifferentTypesofCohortorFollow-UpStudies...63 10 Case-Control Studies...67 Case-CohortSampling ...69 DensitySamplingofControls...69 Case-Non-caseStudy...71 PatientControls ...72 SecondaryIdenti cationoftheSourcePopulation ...74 Case-ControlStudiesUsingPrevalentCases...74 WhentoDoaCase-ControlStudy? ...77 References...78 11 The Cross-Sectional Study...79 12 The Randomized Controlled Trial (RCT) ...81 Reference ...