Managerial Epidemiology
Helpm31990
51589_PPTx_CH03.pptxChapter 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
96.7
X 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%.
