assignment week 1 Managerial Epidemiology

Chapter 3

Measures of Morbidity and

Mortality Used in

Epidemiology

Learning Objectives

• Define and distinguish among ratios,

proportions, and rates

• Explain the term population at risk

• Identify and calculate commonly used

rates for morbidity, mortality, and natality

• State the meanings and applications of

incidence rates and prevalence

Learning Objectives (cont’d)

• Discuss limitations of crude rates and

alternative measures for crude rates

• Apply direct and indirect methods to

adjust rates

• List situations where direct and indirect

adjustment should be used

Overview of Epidemiologic

Measures

Count

• The simplest and most frequently

performed quantitative measure in

epidemiology.

• Refers to the number of cases of a

disease or other health phenomenon

being studied.

Examples of Counts

• Cases of influenza reported in

Westchester County, New York,

during January of a particular year.

• Traffic fatalities in Manhattan in a 24-

hour time period

• College dorm students who had mono

• Foreign-born stomach cancer patients

Ratio

• The value obtained by dividing one

quantity by another.

• Consists of a numerator and a

denominator.

• The most general form has no specified

relationship between numerator and

denominator.

• Rates, proportions, and percentages are

also ratios.

Example of a

Simple Sex Ratio Calculation

• A ratio may be expressed at = X/Y

• Simple sex ratio (data from textbook)

• Of 1,000 motorcycle fatalities, 950 victims

are men and 50 are women.

Number of male cases 950

Number of female cases 50 19:1 male to female= =

Example of a

Demographic Sex Ratio Calculation

• This ratio refers to the number of

males per 100 females. In the U.S.,

the sex ratio in 2010 for the entire

population was 96.7, indicating more

females than males.

Number of male cases 151,781,326

Number of female cases 156,964,212 96.7X 100 = =X 100

Example of a

Sex Ratio at Birth Calculation

• The sex ratio at birth is defined as:

(the number of male births divided by

the number of female births)

multiplied by 1,000.

Number of male births

Number of female births

X 1,000

Definition of Proportion

• A measure that states a count relative

to the size of the group.

• A ratio in which the numerator is part

of the denominator.

• May be expressed as a percentage.

Uses of Proportions

• Can demonstrate the magnitude of a

problem.

• Example: 10 dormitory students

develop hepatitis. How important is

this problem?

– If only 20 students live in the dorm, 50%

are ill.

– If 500 students live in the dorm, 2% are

ill.

Example of a Proportion

• Calculate the proportion of African-

American male deaths among African-

American and white boys aged 5 to 14

years.

Rate

• Definition: a ratio that consists of a

numerator and a denominator and in

which time forms part of the denominator.

• Contains the following elements:

– disease frequency

– unit size of population

– time period during which an event occurs

Crude death rate = Number of deaths in a given year

Reference population

(during the midpoint of the year

X 100,000

Example: Number of deaths in the United States during 2007 = 2,423,712

Population of the U.S. as of July 1, 2007 = 301,621,157

2,423,712

301,621,157 Crude death rate = = 803.6 per 100,000

Example of Rate Calculation

Definition of Prevalence

• The number of existing cases of a

disease or health condition in a

population at some designated

time.

Figure 3-1:

Analogy of

prevalence and

incidence. The

water flowing

down the waterfall

symbolizes

incidence and

water collecting in

the pool at the

base symbolizes

prevalence.

Source: Robert

Friis.

Interpretation of Prevalence

• Provides an indication of the extent

of a health problem.

– Example 1: Prevalence of diarrhea in a

children’s camp on July 13 was 15.

– Example 2: prevalence of obesity

among women aged 55-69 years was

367 per 1,000.

Uses of Prevalence

• Describing the burden of a health

problem in a population.

• Estimating the frequency of an

exposure.

• Determining allocation of health

resources such as facilities and

personnel.

Point Prevalence

Point Prevalence = Number of persons ill

Total number in the group at point in time

Example:

Total number of smokers in the group = 6,234

Total number in the group 41,837

or 14.9%

= 149.0 per 1,000

Period Prevalence = during a time period

Period Prevalence

Number of persons ill

Average population

Example:

Persons ever diagnosed with cancer = 2,293

Average population 41,837 = 5.5%

Definition of Incidence

• The number of new cases of a

disease that occur in a group during a

certain time period.

Incidence Rate (Cumulative

Incidence)

• Describes the rate of development of a

disease in a group over a certain time

period.

• Contains three elements:

– Numerator = the number of new cases.

– Denominator = the population at risk.

– Time = the period during which the cases

occur.

Example of Incidence Data

• Number of new cases of HIV

infection diagnosed in a population

in a given year: a total of 164 HIV

diagnoses were reported among

American Indians or Alaska natives

in the U.S. during 2009.

Incidence Rate Calculation

(IWHS Data)

Incidence rate =

Number of new cases

over a time period

Total population at risk

during the same time period

X multiplier (e.g., 100,000)

Number of new cases = 1,085

Population at risk = 37,105

Incidence rate = 1,085

37,105 = 0.02924/8 = 0.003655 x 100,000

= 365.5 cases per 100,000 women per year

Attack Rate (AR)

• Alternative form of incidence rate.

• Used for diseases observed in a

population for a short time period.

• Not a true rate because time dimension

often uncertain.

• Example: Salmonella gastroenteritis

outbreak

• Formula: Ill

Ill + Well AR = x 100 (during a time period)

Incidence Density

• An incidence measure used when

members of a population or study

group are under observation for

different lengths of time.

Number of new cases during the time period

Total person-time of observation

Incidence

density =

Number of new cases during the time period

Total person-years of observation Incidence density =

If period of observation is measured in years, formula

becomes:

Formulas for Incidence Density

Incidence Density, Example

Interrelationship Between

Prevalence and Incidence

Interrelationship Between

Prevalence and Incidence (cont’d)

• If duration of disease is short and

incidence is high, prevalence becomes

similar to incidence.

• Short duration--cases recover rapidly or

are fatal.

• Example: common cold

Interrelationship Between

Prevalence and Incidence

(cont’d)

• If duration of disease is long and

incidence is low, prevalence

increases greatly relative to

incidence.

• Example: HIV/AIDS prevalence

Crude Rates, Measures of

Natality

• Crude birth rate

• Fertility rate

– General

– Total

• Infant mortality

rate

• Fetal death rate

• Neonatal mortality rate

• Postneonatal mortality

rate

• Perinatal mortality rate

• Maternal mortality rate

Crude Birth Rate

Crude Birth Rate =

Number of live births

within a given period

Population size at the

middle of that period

X 1,000

population

Sample calculation: 4,130,665 babies were born in the U.S.

during 2009, when the U.S. population was 307,006,550.

The birth rate was

4,130,665/307,006,550 = 13.5 per 1,000.

Used to project population changes; it is affected by the

number and age composition of women of childbearing

age

General Fertility Rate

General

fertility rate

=

# of live births

within a year

# of women

aged 15-44 yrs.

during the midpoint

of the year

X 1,000 women

aged 15-44

Sample calculation: During 2009, there were 61,948,144

women aged 15 to 44 in the U.S. There were 4,130,665 live

births. The general fertility rate was 4,130,665/61,948,144 =

66.7 per 1,000 women aged 15 to 44.

Used for comparisons of fertility among age, racial, and

socioeconomic groups.

Total Fertility Rate

• This rate is “[t]he average number of

children that would be born if all women

lived to the end of their childbearing years

and bore children according to a given set

of age-specific fertility rates.”

• In the United States, the total fertility rate

was 2.06 in 2012. This rate is close to

• The replacement fertility rate is 2.1.

Fetal Death Rate

Used to estimate the risk of death of the fetus associated

with the stages of gestation.

Fetal Death Ratio

Refers to the number of fetal deaths after gestation

of 20 weeks or more divided by the number of live

births during a year.

Fetal Death

Ratio =

Number of fetal deaths after

20 weeks or more gestation

Number of live births

X 1,000

(during a

year)

Infant Mortality Rate

Used for international comparisons; a high rate indicates

unmet health needs and poor environmental conditions.

Neonatal Mortality Rate

• Reflects events happening after birth,

primarily: – Congenital malformations

– Prematurity (birth before gestation week

28)

Neonatal Mortality Rate

Formula

Postneonatal Mortality Rate

Measures risk of dying among older infants

during a given year.

Perinatal Mortality Rate

Reflects environmental events that occur during

pregnancy and after birth; it combines mortality during

the prenatal and postnatal periods.

Perinatal Mortality Ratio

Maternal Mortality Rate

Reflects health care access and socioeconomic

factors; it includes maternal deaths resulting from

causes associated with puerperium (period after

childbirth), eclampsia, and hemorrhage.

Crude Rates

• Use crude rates with caution when

comparing disease frequencies between

populations.

• Observed differences in crude rates may

be the result of systematic factors (e.g.,

sex or age distributions) within the

population rather than true variation in

rates.

Specific Rates

• Specific rates refer to a particular

subgroup of the population defined

in terms of race, age, sex, or single

cause of death or illness.

Cause-Specific Rate

Cause-specific mortality rate (age group 25-34) due to HIV in

2003 = 1,588/39,872,598 = 4.0 per 100,000

Example:

Proportional Mortality Ratio

(PMR) %

PMR (%) for HIV among the 25- to 34-year-old group

= 1,588/41,300 = 3.8%

Indicates relative importance of a specific cause of

death; not a measure of the risk of dying of a

particular cause.

Example:

Age-Specific Rate (Ri)

Method for Calculation of Age-

Specific Death Rates

Adjusted Rates

• Summary measures of the rate of

morbidity and mortality in a

population in which statistical

procedures have been applied to

remove the effect of differences in

composition of various populations.

Direct Method

• The direct method may be used if

age-specific death rates in a

population to be standardized are

known and a suitable standard

population is available.

New Standard Population • Year 2000 population

– Replaces the standard based on 1940 population

– Results in age-adjusted death rates that are much

larger

– Affects trends in age-adjusted rates for certain

causes of death

– Narrows race differentials in age-adjusted death

rates

• Reduces the three different standards into

one acceptable standard

Direct Method for Adjustment of

Rates

Weighted Method for Direct Rate

Adjustment

Indirect Method

• Indirect method may be used if age- specific death rates of the population for standardization are unknown or unstable, for example, because the rates to be standardized are based on a small population.

• The standardized mortality ratio (SMR) can be used to evaluate the results of the indirect method.

Standardized Mortality Ratio

(SMR)

Interpretation of SMR

• If the observed and expected numbers are

the same, the SMR would be 1.0,

indicating that observed mortality is not

unusual.

• An SMR of 2.0 means that the death rate

in the study population is two times greater

than expected.

Indirect Age Adjustment

(cont’d)

• From previous table, SMR is

(502/987.9) X 100 = 50.8%.