Measuring Epidemiologic Outcomes

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Measuring Epidemiologic Outcomes
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Epidemiological Outcomes

• Ratio Relationship between two numbers
• Example males/females
• Proportion A ratio where the numerator is
  included in the denominator
• Example males/total births
• Rate A proportion with the specification of time
• Example (deaths in 1999/population in 1999) x
  1,000

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In epidemiology, the occurrence of a disease or
condition can be measured using rates and
proportions. We use these measures to express
the extent of these outcomes in a community or
other population.

• Rates tell us how fast the disease is occurring
  in a population.
• Proportions tell us what fraction of the
  population is affected.
• (Gordis, 2000)

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Morbidity Measures
Number of new events during a time period
X 1,000
Incidence Rate
Population at risk

• Incidence is always calculated for a given period
  of time
• An attack rate is an incidence rate calculated
  for a specific disease for a limited period of
  time during an epidemic

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Morbidity Measures

• Prevalence is not a rate
• Point prevalence measures the frequency of all
  current events (old and new) at a given instant
  in time
• Period prevalence measures the frequency of all
  current events (old and new) for a prescribed
  period of time

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• Interrelationship P ? ID

• High prevalence may reflect
• High risk
• Prolonged survival without cure
• Low prevalence may reflect
• Low risk
• Rapid fatal disease progression
• Rapid cure

Examples Ebola, Common cold
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Relationship Between Incidence and Prevalence
(cont.)

• Cancer of the pancreas
• Incidence low
• Duration short
• Prevalence low
• Adult onset diabetes
• Incidence low
• Duration long
• Prevalence high

• Roseola infantum
• Incidence high
• Duration short
• Prevalence low
• Essential hypertension
• Incidence high
• Duration long
• Prevalence high

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• Calculation Practice

• Skin Cancer on Sunny Beach
• Point prevalence on 9/28/1974
• Period prevalence for year 1975
• Incidence rate for year 1975

What information will you need?
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Diagnosed cases of Skin Cancer on Sunny Beach,
9/28/1974
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of existing cases 10 Total population at risk
450
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Point Prevalence (9/28/1974) (10/450)1000
22 per 1000
?
?
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Diagnosed cases of Skin Cancer on Sunny Beach,
1975 Average population at risk 500
?
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of new cases 5
?
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Incidence rate (year 1975) (5/500)1000
10 per 1000
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Period prevalence (year 1975) (15/500)1000
30 per 1000
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Number of cases of disease beginning, developing,
and ending during a period of time, January 1,
2000 December 31, 2000. Length of each line
corresponds to duration of each case.
JAN 2000
DEC 2000
MAY
JULY
SEPT
What is the numerator for incidence in
2000? What is the numerator for point prevalence
if a survey was done in May? July? September?
December?
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Risk Versus Rate

• Risk and rate are often used interchangeably by
  epidemiologists but there are differences

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Risk Versus Rate (cont.)

• Risk is a probability statement assuming an
  individual is not removed for any other reason
  during a given period of time
• As such, risk ranges from 0 to 1 (no chance to
  100 probability of occurrence)
• Risk requires a reference period and reflects the
  cumulative incidence of a disease over that
  period
• Example 1 in a million chance of developing
  cancer in a 70 year lifetime

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Risk Versus Rate (cont.)

• Rates can be used to estimate risk if the time
  period is short (annual) and the incidence of
  disease over the interval is relatively constant
• If however, individuals are in a population for
  different periods of time for any reason, then
  you should estimate risk by incidence density

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Incidence Density
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ID Example

• In the Iowa Womens Health Study (IWHS), 37,105
  women contributed 276,453 person-years of
  follow-up
• Because there were 1,085 incident cases, the rate
  of breast cancer using the incidence density
  method is
• 1,085/276,453 392.5/100,000 person-years

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ID Example (cont.)

• If each woman had been followed for the entire
  8-year period of the study, the total
  person-years would have been 296,840 and the rate
  would have been lower (assuming the number of
  incident cancers was the same)
• The incidence density method yielded a higher and
  more accurate estimate

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Natality Outcomes

• Natality measures are used primarily by
  demographers for population projection

Number of live births for a given time period
(year)
Crude Birth Rate
X 1,000
Estimated mid-interval total population
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Concerns About Crude Birth Rates

• Definitions of a live birth may vary
• U.S. any product of conception that shows any
  sign of life after complete birth (pulse,
  heartbeat, respiration, crying, pulsation of
  umbilical cord or movement of the voluntary
  muscles)
• The denominator used for birth rates is
  inaccurate because men are not part of the
  population-at-risk

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Natality Outcomes (cont.)
Number of live births for a given time period
(year)
X 1,000
General Fertility Rate
Estimated of women 15-44 years at mid-interval
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Natality Outcomes (cont.)

• Total fertility rate Same as above, but use
  women 10-49 years and adjust for age cohorts
• Gross reproductive rate Same as TFR, but use
  only live births of females in numerator
• Net reproductive rate Same as GRR, but count
  only births of females who survive to
  reproductive age in the numerator

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Net Reproductive Rate (NRR)

• If NRR 1,000, each generation will just replace
  itself
• If NRR lt 1,000, indicates a potentially declining
  population
• If NRR gt 1,000, indicates a potential population
  increase

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Mortality Measures Related to Natality

• Fetal Death Rate or Ratio Used primarily by
  public health officials to estimate the health of
  populations

Fetal Death Rate
Number of fetal deaths 20 weeks or more gestation
in a given interval
X 1,000
Fetal deaths plus live births in that interval
Estimates risk of death associated with late
states of gestation
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Mortality Measures Related to Natality (cont.)
Fetal Death Ratio
Number of fetal deaths 20 weeks or more gestation
in a given interval
X 1,000
Number of live births reported during the same
time interval
Measures fetal loss relative to live births
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Mortality Measures Related to Natality (cont.)
Perinatal Mortality Rate
Number of fetal deaths 20 weeks or more gestation
plus number of neonatal deaths (28 days or
less in age) during a given interval
X 1,000
Number of fetal deaths 20 weeks or more gestation
plus number of live births during the same
interval
Reflects events occurring during pregnancy and
after birth
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Mortality Measures Related to Natality (cont.)
Number of deaths of neonates (28 days or
less) in a given interval
Neonatal Mortality Rate
X 1,000
Number of live births during the
same interval
Estimates events immediately after birth,
primarily congenital malformations, prematurity
and low birth weight
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Mortality Measures Related to Natality (cont.)
Infant Mortality Rate
Number of deaths under 1 year during a given
interval
X 1,000
Number of live births during the same interval
Used for international comparisons high rates
indicate unmet public health needs and poor
socioeconomic and environmental conditions
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Mortality Measures Related to Natality (cont.)
Maternal Mortality Rate
Number of deaths assigned to causes related to
pregnancy during a given interval
X 1,000
Number of live births during the same
interval
Rates reflect health care access and
socioeconomic factors
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Chart of Early Life Mortality Measures
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Mortality Outcomes

• Crude rate
• The number of events in a population over a given
  period of time, usually a calendar year
• Crude rates reflect the probability of an event
• As the probability of death increases with age,
  the crude death rate reflects the age structure
  of the population

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Mortality Outcomes (cont.)
Example 1980
The larger crude death rate in Florida reflects
the larger population of elderly in that state.
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Mortality Outcomes (cont.)

• Specific rate
• Used to construct rates for specific segments of
  the population so we can compare among strata or
  between groups (used especially for age, race,
  ethnicity, gender)
• We can also construct cause-specific rates to
  compare rates among causes

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Mortality Outcomes (cont.)

• Examples
• Age-specific rates
• Gender-specific rates
• Race-specific rates
• Cause-specific rates