Presentation on theme: "Lecture 3: Measuring the Occurrence of Disease"— Presentation transcript:
1 Lecture 3: Measuring the Occurrence of Disease Reading:Gordis – Chapter 3Lilienfeld and Stolley –Chapter 4Chapter 6, pp ,
2 Counting casesOne’s knowledge of science begins when he can measure what he is speaking about and express it in numbers Lord Kelvin ( )To examine the transmission of disease in human populations, we need to be able to measure the frequency of disease occurrence and of deaths from the disease.
3 MeasuresHow do we express the extent of morbidity and mortality resulting from disease?CountsRatioa fraction with no specified relationshipProportionswhat fraction of the population is affectedRateshow fast things are occurring
4 Measures Measures of morbidity Prevalence: a proportion Cumulative incidence: a rateIncidence density: a rateMeasures of mortalityMortality rate: a rateStandardized mortality (SMR)
5 Defining case Natural course of disease Exposure onset symptoms dx outcomeRecoveryDeathChronic diseaseIncubationperiodClinicalStageSubclinicalStage
6 Counts Prerequisite for epidemiologic investigation Simplest measure of disease frequencyFrequency of affected individualsUseful for planning adequacy of health care allocation at a particular levelFor example:Number of West Nile virus cases
7 RatioA fraction with no specified relationship between numerator and denominatorRange: 0 to A/BExamplessex ratio (M:F)
8 Ratio Number of men with syphilis, 1991 2,412 Number of women with syphilis. 19912,314Ratio of male to females2,412/2,314 = 1.04(The numerator is not included in the denominator)
9 Proportion Type of ratio Numerator included in denominator May be expressed as percentagePercentage = proportion x 100 %Range: 0 to 1A/(A+B)ExamplePrevalence
10 Prevalence All individuals with a disease at a given point in time Dimensionless - should not be described as a rate - may be described as a percentnumber of cases (A) todayP =total population (A+B) today
11 PrevalenceProportion of individuals in a population who have the disease or condition of interest at a specific time periodUtilityDescribe health burden of a populationStatus of disease in a populationEstimate the frequency of exposureProject health care needs of affected individuals
12 Types of prevalencePoint prevalence – proportion of all cases at a specific point in timePeriod prevalence – proportion of all cases during a period of time
13 Point and period prevalence Point prevalenceDo you currently have asthma?Period prevalenceHave you had asthma during the last five years?Every person in the numerator had the disease at some time during the period specified.Period prevalence consists of the point prevalence at the beginning of a specified period of time plus all new cases that occur during that period.
14 Rate A special type of proportion Unit of time in denominator A/(A+B) per time intervalAlways two components:New cases and time
15 Incidence Incidence is an important rate… It is the proportion of people (at risk) who develop diseased during a specific time period.Three key elements:Only new cases included in numeratorTotal population at risk in the denominatorTime element – period over which new cases developedTwo main types of Incidence:Cumulative IncidenceIncidence Rate (a.k.a. incidence density)
16 Cumulative IncidenceOne of the most widely used measures of disease risk.Estimate of probability (risk) that an individual will develop disease during a specified period of timeCumulative Incidence =No. of new cases in a given period of timeNo. of people at risk during that time
17 Incidence rate (incidence density) Cumulative Incidence gives each individual equal weight, but different people stay in the study for different length- having different contribution.Measure of the true rate of disease developmentIncidence rate =No. of new cases in a given period of timetotal person-time of observation
18 Person-time 5-year (1/95-1/00) Incidence rate 1/961/971/981/991/00TotalA3BC5D1E4Total years at risk• = enter the study, X = having disease, loss to follow-up16xxxx5-year (1/95-1/00) Incidence rate= 2/16 = 12.5/100 person-years of observation
19 Prevalence vs. incidence 1/951/961/971/981/991/00ABCDE• = enter the study, X = having disease, loss to follow-up, disease developingxxxx1/97-1/00 cumulative incidence cases: A, E1/97-1/00 period prevalence cases: A, D, E1/98 point prevalence: A, D
20 Relationship between prevalence and incidence Incidence is a proxy for “risk”, whereas prevalence is best for assessing disease burden or case load in a geographic area.There is a well known relationship between them, namely –Prevalence = Incidence x Duration of diseaseP = I x D
21 + Incident cases Whole population at time t Prevalent casesPrevalent casesPrevalent casesMinuscures or deaths due to diseasePrevalent cases
22 Examples of P = I x DIf the incidence of diabetes mellitus is 1% per year and its approximate duration is 5 years, then what is its expected point prevalence?Assuming equal incidence of disease, which is more prevalent: pancreatic cancer or brain cancer?Average duration of pancreatic cancer = 3 monthsAverage duration of brain cancer = 1.5 years
23 Measures of mortality Annual mortality rate from all causes = Total no. of death from all causes in 1 yearNo.of people in the population at midyearCase-fatality rate =No. of individuals dying during a specified period of time after disease onsetNo. of individuals with the specified disease
24 Three common forms of rates Crude ratese.g. crude birth rate, crude death rateSpecific ratese.g. sex-specific, age-specific, race-specificAdjusted ratese.g. age-adjusted
25 Crude rate: exampleSuppose County B recorded 4000 births and 1500 deaths in Using U.S. Census data, we find that the population size is 200,000.Crude birth rate =No. of live births in time interval TTotal population= 4,000/200,000 = 20 births per 1,000Crude death rate =No. of deaths in time interval T= 1,500/200,000 = 7.5 deaths per 1,000
26 Specific Rates for Mortality in Older Adults Rates for selected leading causes of death among older adults, by sex, and race -- United States, 1996*Sex RaceCause of death† Total Male Female White BlackHeart disease , , , , ,937(612,199)Malignant neoplasms , , , ,338(382,988)Cerebrovascular diseases(140,448)* MMWR Dec 17, 1999 / 48(SS08);7-25
27 How do we compare rates across populations? Crude rates are not helpful because …Populations differ in their age distributionsPopulations differ in their racial distributionsPopulations differ in their SES distributions
28 How do we compare rates across populations? We compare rates across populations by putting them on an even playing field -that is, we either standardize one population on another orwe use an outside standard and adjust our populations to that standard.
29 For our purposes, the most important is age-adjustment Two types of age-adjustmentDirect MethodIndirect Method (SMR = standard mortality ratio)
30 Direct method: example Population A Population BAGE N Risk Cases N Risk Cases<>CRUDE RISK = 250/800 = 31% 130/800 = 16%Crude risk indicates different risks of disease between populations.But age-specific rates indicate similar risks.
31 Direct method: example Using the total of the two populations as the standard populationPopulation A Population BAGE Std. Risk Cases Std. Risk Casespop. pop.<>AGE-ADJUSTED RISK = 24% 250/800 = 24%
32 Direct method: example Apply risks in population B to population A (using population A as the standard population.Population A Population BAGE Std. Risk Cases Std. Risk Casespop. pop.<>AGE-ADJUSTED RISK = 31% 250/800 = 31%
33 Direct methodWhat information is needed to calculate age-adjusted death rate, using the direct method?Standard population distributed by ageAge-specific death rates in study populationsThe actual value of an age-adjusted rate is meaningless because it depends on the choice of the standard population.It is only meaningful in comparison to other rates which have been adjusted by the same method and the same standard population.
34 Indirect method Apply rates from a standard population to each age stratum in the study populationto obtain expected number.This adjusted rate interpreted as:the rate that would have been experienced by the study population if their rates had been similar to the standard population.
35 Standardized mortality ratio (SMR) Observed number of cases per timeExpected number of cases per timeSMR = 0indicates observed is not unusualSMR > 1.0indicates morbidity (or mortality) exceeds expectedSMR = 2.0 indicates two-fold increaseSMR < 1.0indicates morbidity (or mortality) is less than expected
36 SMR: example- death in white miners Est. pop. of white minersDeath rate in general pop.Expected deathObserved deathAge(1)(2)(3)=(1)X(2)(4)20-247459812.269.141025-298507716.1213.712030-348084521.5417.412235-4414887033.9650.559845-5410264956.8258.3217455-594249475.2331.96112Total534533181.09436SMR = 436/ = 2.41
37 SMR disadvantageSMR produces a ratio instead of a rate. It gives relative information but does not describe the mortality in the population.SMR depends on the choice of the standard population.