chapter 3
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Chapter 3. Measures of Morbidity and Mortality Used in Epidemiology Revised by Susan Bailey, Ph.D. Learning Objectives. Define and distinguish among ratios, proportions, and rates Explain the term population at risk - PowerPoint PPT PresentationTRANSCRIPT
Chapter 3
Measures of Morbidity and Mortality Used in Epidemiology
Revised by Susan Bailey, Ph.D.
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
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.
• Significant for rare diseases or symptom presentations (e.g., case of Ebola virus).
Examples of Counts
• Traffic fatalities in Manhattan in a 24-hour time period
• College dorm students who had hepatitis
• Foreign-born stomach cancer patients
Ratio
• The value obtained by dividing one quantity by another
• The most general form has no specified relationship between numerator and denominator.
• Proportions, rates, and percentages are also ratios.
Example of a Ratio Calculation
• Sex ratio (data from textbook)
Number of male cases 950
Number of female cases 5019:1 male to female= =
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.
A B Total (A + B)
Number of deaths among African-American boys
Number of deaths among white boys
Total
1,150 3,810 4,960
Proportion = A/(A + B) x 100 = (1,150/4,960) x100 = 23.2%
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 2003 = 2,448,288 Population of the U.S. as of July 1, 2003 = 290,810,789
2,448,288 290,810,789
Crude death rate = = 841.9 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.
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 33% (point prevalence).
– Example 2: prevalence of cancer in women during a specified time period (period prevalence)
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.
Applications of Incidence Data
• Help in research on the etiology/causality of disease.
• Used to estimate– the risk of developing a disease.– the effects of exposure to a hypothesized
factor of interest.
Incidence Rate Calculation (IWHS Data)
Incidence rate =
Number of new cases over a time period
Total population at riskduring the same time period
X multiplier (e.g., 100,000)
Number of new cases = 1,085Population at risk = 37,105
Incidence rate = 1,08537,105
= 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
A B A x BNumber of Subjects Length of observation (years) Person-years
30 10 30010 9 907 8 562 7 141 1 1
Total 50 461Number of health events (heart attacks) observed during the 10-year period: 5Incidence density = (5/461) x 100 = 1.08 per 100 person-years of observation
Interrelationship Between Prevalence and Incidence
Interrelationship: P ID=
The prevalence (P) of a disease is proportional to the incidence rate (I) times the duration (D) of a disease.
Interrelationship between Prevalence and Incidence
Incidence
Prevalence
Cure/Death
Prevalence ~Incidence when disease has a short duration (D~0)
High Prevalence could mean• true high incidence or•long duration of disease
•prolonged survival
Low prevalence could mean•true low incidence or•short duration of disease
•rapidly fatal•quick/effective cure
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: many chronic diseases
Outbreak of Meningococcal Infections in a Summer School Class of
10 StudentsStudents Jul 5 Jul 14ABCDEFGHIJDay Su M Tu W Th F Sa Su M Tu W Th F Sa
Duration of Illness
Crude Rates, Measures of Natality
• Crude birth rate• Fertility rate• 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 birthswithin a given period
Population size at themiddle of that period
X 1,000 population
Sample calculation: 4,112,052 babies were born in the U.S. during 2004, when the U.S. population was 293,655,404. The birth rate was4,112,052/293,655,404 = 14.0 per 1,000.
Used to project population changes; it is affected by the numberand age composition of women of childbearing age
General Fertility Rate
Generalfertility rate
=
# of live birthswithin a year
# of womenaged 15-44 yrs.
during the midpointof the year
X1,000 womenaged 15-44
Sample calculation: During 2004, there were 62,033,402 women aged 15 to 44 in the U.S. There were 4,112,052 live births. The general fertility rate was 4,112,052/62,033,402 = 66.3 per 1,000 women aged 15 to 44.
Used for comparisons of fertility among age, racial, and socioeconomic groups.
Fertility rates: United States, 1950-1992. Source: Reprinted from National Center for Health Statistics, Annual Summary of Births, Marriages, Divorces and Deaths, United States, 1992, Monthly Vital Statistics Report, Vol 41, No 13, p. 3. Hyattsville, MD: National Center for Health Statistics; 1993.
Infant Mortality Rate
Infant mortality =
Number of infant deathsamong infants aged 0-365
daysduring the year
Number of live birthsduring the year
X1,000 live
births
Used for international comparisons; a high rate indicates unmethealth needs and poor environmental conditions.
Sample calculation: In the U.S. during 2003, there were 28,025 deaths among infants under 1 year of age and 4,089,950 live births. The infant mortality rate was (28,025/4,089,950) x 1,000 = 6.85 per 1,000 live births.
Fetal Death Rate
Fetal Death Rate (per 1,000 live births plus fetal deaths) =
Number of fetal deaths after 20 weeks or more gestation
Number of live births + number of fetal deathsafter 20 weeks or more gestation
X 1,000
Late fetal death rate (per 1,000 live births plus late fetal deaths) =
Number of fetal deaths after 28 weeks or more gestation
Number of live births + number of fetal deaths after28 weeks or more gestation
X 1,000
Used to estimate the risk of death of the fetus associated with the stages of gestation.
Fetal Death Ratio
Provides a measure of fetal wastage (loss) relative to the number of live births.
Fetal Death Ratio =
Number of fetal deaths after 20 weeks or more gestation
Number of live birthsX 1,000(during a year)
Neonatal Mortality Rate
• Reflects events happening after birth, primarily:– congenital malformations– prematurity (birth before gestation week
28)– low birth weight (birth weight less than
2,500 g)
Neonatal Mortality Rate Formula
Neonatal mortality rate =
Number of infant deaths under 28 days of age
Number of live birthsX 1,000 live births(during a year)
Postneonatal Mortality Rate
Postneonatal mortality rate =
Number of infant deaths from28 days to 365 days after birth
Number of live births - neonatal deaths X 1,000 live births
Reflects environmental events, control of infectious diseases, and improvement in nutrition.
Infant, neonatal, and postneonatal mortality rates: United States, 1940-2003. Source: From Hoyert DL, Heron MP, Murphy SL, Kung H. Deaths: Final Data for 2003. National Vital Statistics Reports, Vol 54, No 13, p. 12. Hyattsville, MD: National Center for Health Statistics; 2006.
Perinatal Mortality Rate
Perinatal mortality rate =
Number of late fetal deaths after28 weeks or more gestation plusinfant deaths within 7 days of birth
Number of live births +number of late fetal deaths
X 1,000 live births and fetal deaths
Reflects environmental events that occur during pregnancy and after birth; it combines mortality during the prenatal and postnatal periods.
Perinatal Mortality Ratio
Number of late fetal deaths after28 weeks or more gestation plusinfant deaths within 7 days of birth
Number of live birthsX 1,000 live births
Perinatal mortality ratio =
Maternal Mortality Rate
Maternal mortality rate (per 100,000 live births, including multiple births) =
Number of deaths assigned to causes related to childbirth
Number of live birthsX 100,000 live births(during a year)
Reflects health care access and socioeconomic factors; it includes maternal deaths resulting from causes associated with pregnancy and puerperium (during and after childbirth).
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 Rate =
Mortality (or frequency of a given disease)
Population size at midpoint of time periodX 100,000
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 (%) =
Mortality due to a specific cause during a time period
Mortality due to all causes during the same time periodX 100
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:
The 10 Leading Causes of Death, 25-34 Years, All Races, Both Sexes, U.S., 2003
(Number in population = 39,872,598Rank Order
Cause of Death Number Proportional Mortality
Ratio (%)
Cause-Specific Death Rate per 100,000
1 Accidents (unintentional injuries) 12,541 30.4 31.5
2 Intentional self-harm (suicide) 5,065 12.3 12.7
3 Assault (homicide) 4,516 10.9 11.3
4 Malignant neoplasms 3,741 9.1 9.4
5 Diseases of the heart 3,250 7.9 8.2
6 HIV disease 1,588 3.8 4.0
7 Diabetes mellitus 657 1.6 1.6
8 Cerebrovascular diseases 583 1.4 1.5
9 Congenital malformations 426 1.0 1.1
10 Influenza and pneumonia 373 0.9 0.9
Mortality for all causes = 41,300
Age-Specific Rate (Ri)
The number of cases per age group of population (during a specified time period).
Number of deaths among those aged 5-14 years
Number of persons who are 5-14 years (during time period) X 100,000
Sample calculation: In the U.S. during 2003, there were 1,651 deaths due to malignant neoplasms among the age group 5 to 14 years, and there were40,968,637 persons in the same age group. The age-specific malignant neoplasm death rate in this age group is (1,651/40,968,637) = 4.0 per 100,000.
Ri =
Example:
Method for Calculation of Age-Specific Death Rates
Age Group
(Years)
Number of Deaths
(Di) In 2003
Number in Population
(Pi) as of July 1, 2003
Age-Specific Rate
(Ri) per 100,000
Under 1 28,025 4,003,606 700.0
1-4 4,965 15,765,673 31.5
5-14 6,954 40,968,637 17.0
15-24 33,568 41,206,163 81.5
25-34 41,300 39,872,598 103.6
35-44 89,461 44,370,594 201.6
45-54 176,781 40,804,599 433.2
55-64 262,519 27,899,736 940.9
65-74 413,497 18,337,044 2,255.0
75-84 703,024 12,868,672 5,463.1
85+ 687,852 4,713,467 14,593.3
Not stated 342 NA NA
Totals 2,448,288 290,810,789 841.9
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.
Don’t confuse age-specific with age-adjusted
Specific –actual rates observed in the population within specific age groups
Number of Deaths for 30-34 year olds
Number of 30-34 year olds in the population
Adjusted – statistically derived rates adjusted by calculating new rates using those from a standard population
Direct Method for Adjustment
• 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
Age Group
(Years)
2003 Age-Specific
Death Rate
(Ri)
Number in
Standard
Population, 2000 (Psi)
Expected Number
of Deaths (Di)
(Ri * Psi)
Under 1 0.007 3,794,901 26,564.08
1-4 .000315 15,191,619 4,784.22
5-14 .00017 39,976,619 6,785.62
15-24 .000815 38,076,743 31,018.66
25-34 .001036 37,233,437 38,566.36
35-44 .002016 44,659,185 90,042.86
45-54 .00433 37,030,152 160,428.66
55-64 .009409 23,961,506 225,462.73
65-74 .02255 18,135,514 408,952.54
75-84 .054631 12,314,793 672,765.23
85+ .145933 4,259,173 621,555.36
Totals ΣPsi=274,633,642 2,286,926.31
Age-adjusted rate per 100,000 = 832.7 (Refer to text.)
Calculation of Weights for Direct Method
• Weights are calculated by dividing the standard population size in each age group by the total standard population size
Wsi = Psi/ΣPsi
for example, the weight for the under age 1 group is
Wunder 1 = 3,794,901/274,633,642 = 0.01381
Weighted Method for Direct Rate Adjustment
Age Group
(Years)
Number in
Standard Population,
2000 (Psi)
Standard
Weight (Wsi) for
2000
Age-Specific
Death Rate
(Ri), 2003
Age- Adjusted Death Rates
Wsi • Ri
Under 1 3,794,901 0.013818 700.0 9.6726
1-4 15,191,619 0.055316 31.5 1.7420
5-14 39,976,619 0.145563 17.0 2.4708
15-24 38,076,743 0.138646 81.5 11.2946
25-34 37,233,437 0.135575 103.6 14.0428
35-44 44,659,185 0.162164 201.6 13.7865
45-54 37,030,152 0.134835 433.2 15.4155
55-64 23,961,506 0.087249 940.9 18.0958
65-74 18,135,514 0.066035 2,255.0 148.9084
75-84 12,314,793 0.044841 5,463.1 244.9682
85+ 4,259,173 0.015509 14,593.3 266.3216
Totals 274,633,642 1.0 N/A 832.7
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)
SMR =Observed deaths
Expected deathsx 100
Sample calculation: The number of observed deaths due to heart diseaseis 600 in a certain county during year 2003. The expected numberof deaths is 1,000. The SMR = (600/1,000 x 100) = 60% (0.6).
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.
Illustration of Indirect Age Adjustment: Mortality Rate Calculation for a Fictitious
Population of 230,109 PersonsAge (Years) Number in
Population of
Interest
(Pi)
Death Rates
in
Standard
Population
(Dsi)
Expected Number
of Deaths in
Population of
Interest
Pi * Dsi
15-24 7,989 .000815 6.5
25-34 37,030 .001036 38.4
35-44 60,838 .002016 122.6
45-54 68,687 .004332 297.6
55-64 55,565 .00949 522.8
Totals 230,109 987.9
Total expected number of deaths = 987.9Observed number of deaths in this population = 502 (given)
Indirect Age Adjustment (cont’d)
• From previous table, SMR is (502/987.9) X 100 = 50.8%.