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%.