2 What common measures are used in the field of epidemiology? What is epidemiology?What common measures are used in the field of epidemiology?What are the subject areas studied by epidemiologists?How has epidemiology evolved over time? What is the current focus of epidemiology?What are the health challenges of modern medicine (and focus of epidemiology)?
3 Definition of Epidemiology Study of the distribution and determinants of diseases and injuries in human populationsConcerned with frequencies and types of injuries and illness in groups of peopleFocus is not on the individualConcerned with factors that influence the distribution of illness and injuries
4 Background Relatively new science – emerged in 19th century Today: In strictest terms – study of epidemicsToday:Concerned with epidemic disease and all other forms of illness and bodily injuryCancer, heart diseaseHIV/AIDSAlcoholism, drug addictionSuicideAutomobile accidents…
5 Relationship Between Clinical Medicine and Epidemiology Focus in medicine is the individual patientCommunity replaces the individual patient in epidemiology
6 Fundamental Assumptions in Epidemiology Disease doesn’t occur at randomDisease has causal and preventive factorsDisease is not randomly distributed throughout a populationEpidemiology uses systematic approach to study the differences in disease distribution in subgroupsAllows for study of causal and preventive factors
7 Components of Epidemiology Measure of disease frequencyQuantification of existence or occurrence of diseaseDistribution of disease - three questionsWho is getting disease?Where is disease occurring?When is disease occurring?Formulation of hypotheses concerning causal and preventive factorsDeterminants of diseaseHypothesis are tested using epidemiologic studies
8 Progression of Epidemiologic Reasoning 1. Suspicion that a factor may influence occurrence of diseaseObservations in clinical practiceAre HC providers seeing unexpected illness patterns in their patients?Examination of disease patternsDo subpopulations have higher or lower rates?Are disease rates increased in the presence of certain factors?Observations in laboratory researchTheoretical speculationWhat theories can be generated from existing knowledge of disease prevention and causation models?
9 Progression of Epidemiologic Reasoning (Cont.) 2. Formulation of specific hypothesesBased on suspicions concerning influence of a particular factor on disease occurrence3. Conduct studyHypotheses are tested to determine if statistical associations between factors and disease occurrence existStudy population is assembled from individuals with disease or outcome of interest and an appropriate comparison groupData is collected and analyzed
10 Progression of Epidemiologic Reasoning (Cont.) 4. Assess validity of associationDoes the observed association really exist?Is the association valid?Are there alternative explanations for the association?ChanceBiasConfounding
11 Progression of Epidemiologic Reasoning (Cont.) 5. Make a judgement of whether a cause-effect relation between factor (exposure) existsWhat is the magnitude of the association?Are the findings consistent with previous studies (or conflicting)?Are the findings biologically credible?Can underlying biological mechanisms that support the association be identified?
12 Historical Perspective Hippocrates - 5th centuryAssociation between external environment and personal characteristics and health
13 “Whoever wishes to investigate medicine properly should proceed thus: in the first place consider the seasons of the year, and what effects each of them produces. Then the winds, the hot and the cold, especially such are as common to all countries, and then such as are peculiar to each locality. In the same manner, when one comes into a city to which he is a stranger, he should consider its situation, how it lies as to the winds and the rising of the sun; for it influence is not the same whether it lies to the north or the south, to the rising or to the setting sun. One should consider most attentively the waters which the inhabitants use, whether they be marshy and soft, or hard and running form elevated and rocky situations, and then if saltish and unfit for cooking; and the ground, whether it be naked and deficient in water, or wooded and well watered, and whether it lies in a hallow, confined situation, or is elevated and cold; and the mode in which the inhabitants live, and what are their pursuits, whether they are fond of drinking and eating to excess, and given to indolence, or are fond of exercise and labor.” (Hippocrates, “On airs, waters and places” Medical Classics 3:19, 1938).
14 Historical Perspective John Graunt – 1662 (Hennekins and Buring 1987)The Nature and Political Observations Made Upon the Bills of MortalitySystematic statistical approachAnalyzed births and deaths in LondonExcess of males born, higher mortality for malesInfant mortality is very highSeasonal variation for mortalityImportance of routinely collected information for study of human illnessWilliam FarrExamined mortality and occupation and marital statusIdentified important issues in epidemiological investigationsUse of comparison population, influence of multiple factors on disease
15 Historical Perspective John Snow (1854) – Father of modern epidemiologyEstablished modern epidemiologic methodsCholera epidemic in LondonPlotted geographical location of all cases – deaths from cholera
16 From The Visual Display of Quantitative Data, Edward R. Tufte
17 John Snow (cont)Went door to door, collecting information on daily habitsSuspected water supply as source of epidemicBroad street pump closed, epidemic stoppedMode of investigation – “shoe leather”Practical application of epidemiology – use epidemiological investigation to impact a health problem
18 How the Epidemiologist Works Studies origin and distribution of a health problemCollection of dataConstructs a logical chain of inferences to explain the various factors in a society or segment of society that cause a health problem to existLikened to a detective investigating the scene of a crime looking for cluesStarts with examination of sick person(s)Extends investigation to the setting where illness is occurringLooks for common denominator that links all the affected so that the cause of the problem can be eliminated or controlled
19 Epidemiologic Analyses – Areas of Study Causal agents related to disease:Biological agents – bacteria, viruses, insectsNutritional agents – diet (fats, carbohydrates, food nutrients)Chemical agents – gases, toxic agentsPhysical agents – climate, vegetation, chemical pollutants (air, water, food)Social agents – occupation, stress, social class, lifestyle, location of residence
20 Epidemiologist studies: Host characteristics:Biological factorsAge, sex, degree of immunity, other physical attributes that promote resistance or susceptibilityBehavioral factorsHabits, culture, lifestyleSocial environmentLiving conditions such as poverty, crowdingNorms, values and attitudesSocially prescribed standards of livingUse of food and water, food handling practicesHousehold and personal hygiene
21 Eras of Epidemiology Sanitary era – early 19th century Infectious disease era – between late 19th century and early 20th centuryChronic disease era – 2nd half of 20th centuryEco-epidemiology era – 21st century
23 Case Episode of disorder, illness, or injury affecting an individual Case of measlesCancer caseTB caseFood poisoning eventVarious sources provide case informationInterviews or surveysMedical providersInstitutions or agencies
24 IncidenceMeasure of new cases of disease (or other events of interest) that develop in a population during a specified period of timeE.g. Annual incidence, five-year incidenceMeasure of the probability that unaffected persons will develop the diseaseUsed when examining an outbreak of a health problem
25 Prevalence Number of existing cases of disease or other condition Proportion of individuals in a population with disease or condition at a specific point of timeDiabetes prevalence, smoking prevalenceProvides estimate of the probability or risk that one will be affected at a point in timeProvides an idea of how severe a problem may be – measures overall extentUseful for planning health services (facilities, staff)
26 Epidemic, Endemic and Pandemic Any significant increase in the number of persons affected by a diseaseThe first occurrence of a new diseaseEndemicA disease that is established within a population that remain at a fairly stable prevalencePandemicWidespread, universal disease penetration over a wide geographic area
27 More Terms Morbidity – illnesses, symptoms, impairments Mortality - deathsAcute disease – diseases that strike and disappear quickly, within a month or so (chicken pox, colds)Chronic disease – long term or lifelong diseases, incurable
28 More Terms Birth cohort Life expectancy (LE) Persons born in a given yearLife expectancy (LE)Average number of years of life remaining to a person at a particular ageBased on mortality rates and personal characteristics (e.g. gender, race)Years of potential life lost (YPLL)Measure of premature mortalityDeath before age 75
30 1:1.1 ratio of female to male births Used to compare two quantities1:1.1 ratio of female to male birthsUsed to show quantity of disease in a populationcasespopulation
31 ProportionA specific type of ratio in which the numerator is included in the denominator, usually presented as a percentage
32 Calculation of proportion: Males undergoing bypass surgery at Hospital ATotal patients undergoing bypass surgery at Hospital A352 males undergoing bypass surgery539 total patients undergoing bypass surgery65.3%=
33 Rate Special form of proportion that includes a specification of time Most commonly used in epidemiology because it most clearly expresses probability or risk of disease or other events in a defined population over a specified period of time3 major typesCrude ratesSpecific rates (age-specific, infant mortality)Adjusted rates
34 Crude rates Unadjusted, simple ratios Crude mortality rate: cases in defined period of timex Kpopulation in defined period of time(k denotes units 100’s, 1,000, etc.)Crude mortality rate:Total deaths in 2003x 1,000 = U.S. death rateEstimated U.S. pop in 2003
35 Calculation of rates: Number of events in a specified time period Population at risk of these events in a specified time periodX kk is used to denote the units of population such as per 1,000 or per 100,0009,981 deaths in Detroit in 2000951,270 total population in Detroit 2000= per 1,0001049 per 100,000
36 Detroit Population N=951,270 in 2000 9981 deaths15,892 births7,181 to single “named” parentIs Detroit population declining, stable or increasing?
37 Specific RatesCapture effects of specific variables or social characteristicsAge-specific, gender-specific, gender and race-specificExample – infant mortality – deaths within the 1st year of lifeTotal # of deaths in 2003 amongpersons age less than 1 yearx 1,000 = infantNumber of live births during mortality rate
39 Adjusted or Standardized Rates Allow for comparison of populations with different characteristicsStatistically constructed summary rates allow for appropriate comparisons by taking into account differences in populations (age, gender, etc.)Example of use: Population in Arizona is much older than population in Alaska, so it would be inappropriate to compare mortality rates. Standardization allows for meaningful comparisons.
40 Calculating prevalence: Number of existing cases of diseaseP = at a given point in timeTotal population at risk2176 DNW pts with asthma encounterP = = .0731005 DNW pts= 7 asthmatics per 100 pts= 7 %
41 Prevalence calculation exercise: Pediatric Asthma at DNW Number of existing cases of diseaseP = at a given point in timeTotal population at risk2159 DNW pts < 19 with asthma encounterP = =9173 DNW pts < 19=
42 Types of PrevalencePoint prevalence: number of cases that exist at a given point in timeLifetime prevalence: proportion of the population that has a history of a given disorder at some point in timePeriod prevalence: number of cases that exist in a population during a specified period of time
43 Cumulative IncidenceThe proportion of individuals who become diseased during a specified time period.Time period can be a calendar year, 6 months, 3 years, 5 years, etc.
44 Formula for cumulative incidence: Number of new cases of disease during a given time periodCI =Total population at risk70 new cases of breast cancer in a 5 year periodCI =3,000 women at risk== 23 cases per 1,000 women during 5 years
45 Incidence Rate Also known as incidence density Measure of incidence that is able to handle varying observation periodsDenominator is sum of person-time at risk
46 Formula for incidence rate or incidence density: Number of new cases of disease during a given time periodID =Total person-time at risk70 new cases of breast cancerID =13,000 women-years of observation== 5.4 cases / 1,000 women years
47 Relationship Between Incidence and Prevalence Prevalence varies directly with both incidence and duration.If incidence is low, but duration is long (chronic), prevalence will be large in relation to incidence.If prevalence is low because of short duration (due to recovery, migration or death), prevalence will be small in relation to incidence.
48 Special Types of Incidence Rates Morbidity rate - number of nonfatal cases in the population at risk during a specified period of timeMortality rate - number of deaths in a population at risk during a specified period of timeCause-specific mortality - death from a specific causeCase fatality rate - number of deaths from a disease divided by all case of the diseaseAttack rate - cumulative incidence expressing the risk of disease among a population observed for a specified period of time
49 Special types of incidence and prevalence measures RateTypeNumeratorDenominatorMorbidity rateIncidenceNew cases of nonfatal diseaseTotal population at riskMortality rateNumber of deaths from a disease or all causesTotal populationCase-fatality rateNumber of deaths from a diseaseNumber of cases of that diseaseAttack rateNumber of cases of a diseaseTotal population at risk, for a limited period of observationDisease rate at autopsyPrevalenceNumber of persons autopsiedBirth defect rateNumber of babies with a given abnormalityNumber of live births
50 Measures of Association Calculations used to measure disease frequency relative to other factorsIndications of how more or less likely one is to develop disease as compared to another
51 Two by Two TablesUsed to summarize frequencies of disease and exposure and used for calculation of association.DiseaseYesNoTotalabYesa + bExposurecdNoc + dTotala + cb + da + b + c + d
52 Two by Two Tables: Contents of Cells a = number of individuals who are exposed and have the diseaseb = number who are exposed and do not have the diseasec = number who are not exposed and have the diseased = number who are both non-exposed and non-diseased***************************************************a + b = the total number of individuals exposedc + d = the total number of unexposeda + c = the total number with the diseaseb + d = the total number without the diseasea + b + c + d = sum of all four cells and the total sample size for the study
53 Relative RiskMeasure of association between incidence of disease and factor being investigatedRatio of incidence rate for persons exposed to incidence rate for those not exposedIncidence rate among exposedRR =Incidence rate among unexposedEstimate of magnitude of association between exposure and disease
54 Formula for relative risk: Incidence rate among exposedRR =Incidence rate among unexposeda / (a + b)RR =c / (c+ d)Risk ratioIf RR calculated from cumulative incidenceRate ratioIf RR calculated from incidence rate (person units of time)
55 RISK RATIO: Example Breast No Breast Cancer Cancer Total Alcohol , ,000No alcohol , ,000RR using Cumulative Incidence (CI):a/(a + b) / 3,000c/(c + d) / 3,000== 1.4=
56 Interpretation of Relative Risk 1 = No association between exposure and diseaseIncidence rates are identical between groups> 1 = Positive association< 1 = Negative association or protective effectExample: .5 = half as likely to experience disease
57 Odds Ratio Breast No Breast Cancer Cancer Alcohol 70 100 No alcohola x d (70) (140)b x c (50) (100)* Used for case control studies because persons are selected based on disease status so you can’t calculate risk of getting diseaseOR === 2.0
58 Difference Measures Attributable risk # of cases among the exposed that could be eliminated if the exposure were removed= Incidence in exposed - Incidence in unexposedPopulation attributable risk percentProportion of disease in the study population that could be eliminated if exposure were removedIncidence in total population - Incidence in unexposedincidence in total population=
59 Impact of Modernization on Health Infant mortality decreasedLife expectancy greatly increased during 20th centuryMalesIncreased from 48 to 74 yearsFemalesIncreased from 51 to 79 yearsPersons living longer with multiple illnessesChronic and degenerative diseasesIllness with social causes requiring social solutions
61 U. S. Life Expectancy Remaining life expectancy in years Males Females 1900 – at birth46.348.71950 – at birth65.671.11950 – at age 6512.815.81999 – at birth73.979.41999 – at age 6516.119.11999 – at age 7510.012.1Health, United States, 2002
62 U.S. Infant Mortality Rate Deaths per 1,000 infantsSignificance:Measure of society’s sanitary and medical standardsHealth careDietLiving conditions
63 Factors Influencing Changing Pattern Improvements due to industrializationNutritionEnvironmentalSanitationWater supplyHousingMedical advancementsAntibioticsImmunizationDisease surveillance programs
64 Factors Influencing Changing Patterns Problems associated with industrializationEnvironmental pollutantsIncrease in smokingExcess consumption of calories and dietary fatsLack of exercise, physical activityStressAlcohol, drug use
65 Challenges of Modern Medicine Behavioral aspects of healthPromotion of healthy lifestylesDietExerciseTobacco, alcohol, drugsSexual behaviorManagement of stressNew diseases – AIDS, SARS, West Nile Virus, bioterrorism