Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley...
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Transcript of Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley...
![Page 1: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/1.jpg)
Lecture 3:Measuring the Occurrence of
Disease
Reading:Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117
![Page 2: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/2.jpg)
Counting cases
• One’s knowledge of science begins when he can measure what he is speaking about and express it in numbers Lord Kelvin (1824-1907)
• 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.
![Page 3: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/3.jpg)
Measures
• How do we express the extent of morbidity and mortality resulting from disease?– Counts– Ratio
• a fraction with no specified relationship– Proportions
• what fraction of the population is affected
– Rates• how fast things are occurring
![Page 4: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/4.jpg)
Measures
• Measures of morbidity– Prevalence: a proportion– Cumulative incidence: a rate– Incidence density: a rate
• Measures of mortality– Mortality rate: a rate– Standardized mortality (SMR)
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Defining case
Natural course of disease
Exposure onset symptoms dx outcome
Incubationperiod
SubclinicalStage
ClinicalStage
RecoveryDeath
Chronic disease
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Counts
• Prerequisite for epidemiologic investigation• Simplest measure of disease frequency
– Frequency of affected individuals• Useful for planning adequacy of health care
allocation at a particular level • For example:
– Number of West Nile virus cases
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Ratio
• A fraction with no specified relationship between numerator and denominator
• Range: 0 to • A/B• Examples
– sex ratio (M:F)
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Ratio
• Number of men with syphilis, 19912,412
• Number of women with syphilis. 19912,314
• Ratio of male to females2,412/2,314 = 1.04
(The numerator is not included in the denominator)
![Page 9: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/9.jpg)
Proportion
• Type of ratio• Numerator included in denominator• May be expressed as percentage
Percentage = proportion x 100 %• Range: 0 to 1• A/(A+B)• Example
– Prevalence
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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 percent
number of cases (A) todayP =
total population (A+B) today
![Page 11: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/11.jpg)
Prevalence
• Proportion of individuals in a population who have the disease or condition of interest at a specific time period
• Utility– Describe health burden of a population– Status of disease in a population– Estimate the frequency of exposure– Project health care needs of affected
individuals
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Types of prevalence
• Point prevalence – proportion of all cases at a specific point in time
• Period prevalence – proportion of all cases during a period of time
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Point and period prevalence• Point prevalence
– Do you currently have asthma?• Period prevalence
– Have 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.
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Rate
• A special type of proportion• Unit of time in denominator• A/(A+B) per time interval• Always two components:
– New cases and time
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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 numerator– Total population at risk in the denominator– Time element – period over which new
cases developed
• Two main types of Incidence:
– Cumulative Incidence
– Incidence Rate (a.k.a. incidence density)
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Cumulative Incidence
• One of the most widely used measures of disease risk.
• Estimate of probability (risk) that an individual will develop disease during a specified period of time
• Cumulative Incidence =
No. of new cases in a given period of time
No. of people at risk during that time
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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 development
• Incidence rate =No. of new cases in a given period of time
total person-time of observation
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Person-time
ID 1/95 1/96 1/97 1/98 1/99 1/00 Total
A 3
B 3
C 5
D 1
E 4
Total years at risk
• = enter the study, X = having disease,
loss to follow-up
16
x
x
5-year (1/95-1/00) Incidence rate = 2/16 = 12.5/100 person-years of observation
x
x
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Prevalence vs. incidenceID 1/95 1/96 1/97 1/98 1/99 1/00
A
B
C
D
E
• = enter the study, X = having disease,
loss to follow-up, disease developing
x
x
x
x
1/97-1/00 cumulative incidence cases: A, E1/97-1/00 period prevalence cases: A, D, E1/98 point prevalence: A, D
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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 disease
P = I x D
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Prevalent cases
+ Incident cases
Whole population at time t
Prevalent cases
Prevalent cases
Prevalent cases
Minus
cures or deaths due to disease
Whole population at time t +1
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Examples of P = I x D • If 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 months
Average duration of brain cancer = 1.5 years
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Measures of mortality
• Annual mortality rate from all causes =
Total no. of death from all causes in 1 year
No.of people in the population at midyear• Case-fatality rate =
No. of individuals dying during a specified period of time after disease onset
No. of individuals with the specified disease
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Three common forms of rates
• Crude rates
e.g. crude birth rate, crude death rate
• Specific rates
e.g. sex-specific, age-specific, race-specific
• Adjusted rates
e.g. age-adjusted
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Crude rate: exampleSuppose County B recorded 4000 births and 1500 deaths in 1999. Using U.S. Census data, we find that the population size is 200,000.
Crude birth rate =
No. of live births in time interval T
Total population
= 4,000/200,000 = 20 births per 1,000
Crude death rate =
No. of deaths in time interval T
Total population
= 1,500/200,000 = 7.5 deaths per 1,000
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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 Race Cause of death† Total Male Female White BlackHeart disease 1,808 1,983 1,686 1,820 1,937(612,199)Malignant neoplasms 1,131 1,442 915 1,125 1,338(382,988) Cerebrovascular diseases 415 374 443 412 479(140,448)
* MMWR Dec 17, 1999 / 48(SS08);7-25
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How do we compare rates across populations?
Crude rates are not helpful because …
Populations differ in their age distributions
Populations differ in their racial distributions
Populations differ in their SES distributions
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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 or
we use an outside standard and adjust our populations to that standard.
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For our purposes, the most important is age-adjustment
Two types of age-adjustment
Direct Method
Indirect Method (SMR = standard mortality ratio)
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Direct method: example
Population A Population B
AGE N Risk Cases N Risk Cases
<20 100 .1 10 500 .1 5021-50 200 .2 40 200 .2 40>50 500 .4 200 100 .4 40 800 250 800 130
CRUDE RISK = 250/800 = 31% 130/800 = 16%
• Crude risk indicates different risks of disease between populations.
• But age-specific rates indicate similar risks.
![Page 31: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/31.jpg)
Direct method: example
Using the total of the two populations as the standard population
Population A Population B
AGE Std. Risk Cases Std. Risk Casespop. pop.
<20 600 .1 60 600 .1 6021-50 400 .2 80 400 .2 80>50 600 .4 240 600 .4 240
1600 380 1600 380
AGE-ADJUSTED RISK = 24% 250/800 = 24%
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Direct method: exampleApply risks in population B to population A (using population A as the standard population.
Population A Population B
AGE Std. Risk Cases Std. Risk Casespop. pop.
<20 100 .1 10 100 .1 1021-50 200 .2 40 200 .2 40>50 500 .4 200 500 .4 200
800 250 800 250
AGE-ADJUSTED RISK = 31% 250/800 = 31%
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Direct method• What information is needed to calculate age-
adjusted death rate, using the direct method?– Standard population distributed by age– Age-specific death rates in study
populations• The 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.
![Page 34: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/34.jpg)
Indirect method
• Apply rates from a standard population
to each age stratum in the study population
to 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.
![Page 35: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/35.jpg)
Standardized mortality ratio (SMR)• SMR =
Observed number of cases per timeExpected number of cases per time
• SMR = 0 – indicates observed is not unusual
• SMR > 1.0 – indicates morbidity (or mortality) exceeds
expected• SMR = 2.0 indicates two-fold increase
• SMR < 1.0– indicates morbidity (or mortality) is less
than expected
![Page 36: Lecture 3: Measuring the Occurrence of Disease Reading: Gordis – Chapter 3 Lilienfeld and Stolley – Chapter 4 Chapter 6, pp. 101-105, 109-117.](https://reader035.fdocuments.in/reader035/viewer/2022062222/56649e055503460f94af1c79/html5/thumbnails/36.jpg)
SMR: example- death in white miners
Est. pop. of white miners
Death rate in general pop.
Expected death
Observed death
Age (1) (2) (3)=(1)X(2) (4)
20-24 74598 12.26 9.14 10
25-29 85077 16.12 13.71 20
30-34 80845 21.54 17.41 22
35-44 148870 33.96 50.55 98
45-54 102649 56.82 58.32 174
55-59 42494 75.23 31.96 112
Total 534533 181.09 436
SMR = 436/181.09 = 2.41
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SMR disadvantage
• SMR 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.