DESCRIPTIVE EPIDEMIOLOGYDESCRIPTIVE EPIDEMIOLOGYfor Public Health Professionals for Public Health Professionals

Part 3Part 3Ian R.H. Rockett, PhD, MPH

Department of Community Medicine

West Virginia University School of Medicine

Prepared under the auspices of the Prepared under the auspices of the Southeast Public Health Training Southeast Public Health Training Center, University of North Carolina, Center, University of North Carolina, Chapel Hill, 2005.Chapel Hill, 2005.

irockett@hsc.wvu.edu

MEASUREMENTMEASUREMENT

Health is a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity

World Health Organization, 1948

Purposes of Health-related Measurement

Disease and Injury Prevention

Health Promotion

Health Services Planning and Intervention Design

Evaluation

Measuring MortalityMeasuring Mortality

Crude Death RateCrude Death Rate

TheThe DEATH RATEDEATH RATE (more correctly, the (more correctly, the Crude Crude Death RateDeath Rate) can be measured as the number of ) can be measured as the number of deaths per 100,000 population in a given year.deaths per 100,000 population in a given year.

# of deaths# of deaths

Total midyear Total midyear populationpopulation

** kk = =471,000471,000

67,300,00067,300,000 ** 100,000 = 700100,000 = 700

In 2002, the death rate in Turkey was 700 per 100,000 In 2002, the death rate in Turkey was 700 per 100,000 population.population.

In the same year, Guinea’s death rate was 1,724/100,000 In the same year, Guinea’s death rate was 1,724/100,000 population and Singapore’s rate was 400/100,000 population.population and Singapore’s rate was 400/100,000 population.

Age-Specific Death RateAge-Specific Death RateDeath rates can be calculated for specific age groups in Death rates can be calculated for specific age groups in order to compare mortality across different ages or at the order to compare mortality across different ages or at the same age or in the same age group over time. same age or in the same age group over time. Comparisons also can be made across countries or other Comparisons also can be made across countries or other political or geographic entities.political or geographic entities.Since mortality can vary considerably by sex, race, and Since mortality can vary considerably by sex, race, and ethnic group, separate age-specific death rates are often ethnic group, separate age-specific death rates are often presented for males and females, and for different presented for males and females, and for different racial/ethnic groups.racial/ethnic groups.

Deaths of people ages Deaths of people ages 35 - 4435 - 44

Total midyear Total midyear population ages 35-44population ages 35-44

** kk = =663663

272, 249272, 249 ** 100,000 = 244 100,000 = 244

In West Virginia in 2000, the age-specific death rate for persons ages In West Virginia in 2000, the age-specific death rate for persons ages 35-44 was 244 per 100,000 population in those ages.35-44 was 244 per 100,000 population in those ages.

By comparison, the corresponding age-specific death rate in the By comparison, the corresponding age-specific death rate in the United States’ population was 200 per 100,000 people.United States’ population was 200 per 100,000 people.

Cause-Specific Death RateCause-Specific Death Rate

Cancer deathsCancer deaths

Mid-year total Mid-year total populationpopulation

k ;k ;553,091553,091

275,264,999275,264,999 100,000 = 200.9100,000 = 200.9

Expressed as deaths per 100,000 for most causes of xpressed as deaths per 100,000 for most causes of death, but sometimes per 1,000,000 when rates of death, but sometimes per 1,000,000 when rates of occurrence are extremely low.occurrence are extremely low.

In 2000, 201 persons per 100,000 population died of In 2000, 201 persons per 100,000 population died of cancer in the United Statescancer in the United States..

** **

Proportionate Mortality RateProportionate Mortality Rate

# of deaths from # of deaths from cancercancer

Total deathsTotal deathsk k = =

553, 091553, 091

2, 403, 3512, 403, 351 100 100 = = 23 % 23 %

Deaths from a specific cause can be expressed Deaths from a specific cause can be expressed as a percentage of all deathsas a percentage of all deaths

In 2000, 23% of all deaths in the United States were In 2000, 23% of all deaths in the United States were attributable to cancer.attributable to cancer.

* *

Maternal Mortality RatioMaternal Mortality RatioThe The maternal mortality ratiomaternal mortality ratio is the number of women who is the number of women who die as a result of complications of pregnancy or die as a result of complications of pregnancy or childbearing in a given year per 100,000 live births in that childbearing in a given year per 100,000 live births in that year. Deaths due to complications of spontaneous or year. Deaths due to complications of spontaneous or induced abortions are included.induced abortions are included.

# of maternal # of maternal deathsdeaths

Total live births Total live births ** k k ==

185185

1,408,1591,408,159** 1,000 1,000 = = 13.1 13.1

There were 13 maternal deaths per 100,000 live births in There were 13 maternal deaths per 100,000 live births in Russia in 1994.Russia in 1994.

This measure is sometimes referred to as the maternal This measure is sometimes referred to as the maternal mortality rate. mortality rate.

Measuring Morbidity

Prevalence and

Incidence

Prevalence

Prevalence measures the number of cases (new and old) of the disease (or other health-related phenomenon) at a point or period in time

Prevalence (not actually a rate as it ignores the duration of exposure to

the hazard; that is, the time dimension)

Number of cases of a disease present in the population at a specified time

Number of persons in the population at that specified time

* 100

Point Prevalence (as a percentage) =

Incidence

Incidence measures the number of new cases of a disease (or other health-related phenomenon) that occur during a specified period of time in a population at risk

Accumulation of person-years observed in an eight-year cohort study of six subjects

Death

Disease

Death

Disease

Lost to Follow-up

Alive

I

II

III

IV

V

VI

Time (years)0 6 7 83 4 51 2

Study starts Study ends

6

7

8

4

4

5

34

Person-years of Observation

SSubject

Incidence Rate

Number of new events in specified period

Person-time exposure to risk during this period

* k

RelationshipRelationship between between IncidenceIncidence

and and

PrevalencePrevalence

Incident (new) cases

Prevalent (existing) cases

Deaths/ Recoveries/Out-Migrants

Source:http://www.letspaintrunaways.com/images/ Double-waterfall-Sioux-quar.jpg

Factors Influencing Observed Disease

Prevalence

Longer duration of the disease

Increase in new cases (increased incidence)

In-migration of cases

Out-migration of noncases

In-migration of susceptible people

Higher case-fatality rate of disease

Decrease in new cases (decreased incidence)

In-migration of noncases

Out-migration of cases

Improved cure rate of cases

Improved diagnostic facilities (better reporting)

Prolongation of life of patients without care

Shorter durationof the disease

Out-migration of susceptible people

Mortalitymeets

Morbidity

Case-Fatality Rate

Number of deaths due to a disease

Number of people with the same disease

Example: – 600 people have disease– 9 of them die – CFR = (9/600) 100 = 1.5 %

100*

*

Example: Infant Mortality RateThe Infant Mortality Rate is the number of deaths of infants (that is, children less than age one) per 1,000 live births in a given year.

# of deaths of infants under age 1 in a given year

Total live births in that year

k =27,960

4,058,882 1,000 = 24.6

There were an estimated 25 deaths of infants per 1,000 live births in Venezuela in 2002. The lowest estimated rate in that year was for Sweden, at 3.4 per 1,000 live births. A very high national rate would be Angola’s,

estimated at 192 per 1,000 live births in 2002.

* *

Selected Rounded Annual Crude Death Rates per 100,000

population, 2001

Algeria

Mexico

The Netherlands

United States

600

500

900

900

Population Pyramids

United States Population, 1995

Source: Joseph A. McFalls, Jr. Population: A Lively Introduction. Population Bulletin 46(2); 1995: 22.

Population PyramidReally an age-sex pyramid. Can be graphed in

two ways:

1) in absolute numerical terms

2) (better) as percentage distribution - gives comparability across time and space

N = (males + females) 100%

Bases for Comparison

1) variation across age groups

2) variations within age group by sex

Source: Joseph A. McFalls, Jr. Population: A Lively Introduction. Population Bulletin 46(2); 1995: 22.

Population Pyramid of Germany, 1996

Source: Joseph A. McFalls, Jr. Population: A Lively Introduction. Third edition. Population Bulletin 58(4); 2003: 28.

Source: Ian R.H. Rockett. Population and Health: An Introduction to Epidemiology. Second edition. Population Bulletin 54(4); 1999: 17.