2. Epidemiology (Schneider)
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
3. Epidemiology (Schneider)
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)
4. Epidemiology (Schneider)
Morbidity Measures
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
Population at risk
X 1,000
Number of new
events during a time
period
Incidence Rate =
5. Epidemiology (Schneider)
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
Population at risk
X 1,000
Number of existing
events, old and new
Prevalence =
6. Epidemiology (Schneider)
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
7. Epidemiology (Schneider)
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
8. Epidemiology (Schneider)
Calculation Practice
Skin Cancer on Sunny Beach:
1. Point prevalence on 9/28/1974
2. Period prevalence for year 1975
3. Incidence rate for year 1975
What information will you need?
10. Epidemiology (Schneider)
Diagnosed cases of Skin Cancer
on Sunny Beach, 1975
Average population at risk = 500
Incidence rate (year 1975)
= (5/500)*1000
= 10 per 1000
Period prevalence (year
1975)
= (15/500)*1000
= 30 per 1000
# of new cases = 5
11. Epidemiology (Schneider)
JAN
2000
MAY JULY SEPT
DEC
2000
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?
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.
13. Epidemiology (Schneider)
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
14. Epidemiology (Schneider)
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
16. Epidemiology (Schneider)
ID Example
In the Iowa Women’s 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
17. Epidemiology (Schneider)
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
18. Epidemiology (Schneider)
Natality Outcomes
Natality measures are used primarily by
demographers for population projection
Estimated mid-interval total
population
X 1,000
Number of live births
for a given time period (year)
Crude Birth Rate =
19. Epidemiology (Schneider)
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
20. Epidemiology (Schneider)
Natality Outcomes (cont.)
Estimated # of women 15-
44 years at mid-interval
X 1,000
Number of live births for a
given time period (year)
General Fertility Rate =
21. Epidemiology (Schneider)
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
22. Epidemiology (Schneider)
Net Reproductive Rate (NRR)
If NRR = 1,000, each generation will just
replace itself
If NRR < 1,000, indicates a potentially
declining population
If NRR > 1,000, indicates a potential
population increase
23. Epidemiology (Schneider)
Mortality Measures Related to Natality
Fetal Death Rate or Ratio: Used primarily by public
health officials to estimate the health of populations
Estimates risk of death associated with late states of gestation
Fetal deaths plus live births in
that interval
X 1,000
Number of fetal deaths 20 weeks or
more gestation in a given intervalFetal Death
Rate =
24. Epidemiology (Schneider)
Mortality Measures Related
to Natality (cont.)
Measures fetal loss relative to live births
Number of live births reported
during the same time interval
X 1,000
Number of fetal deaths 20 weeks or
more gestation in a given intervalFetal Death
Ratio =
25. Epidemiology (Schneider)
Reflects events occurring during pregnancy and after birth
Number of fetal deaths 20 weeks or
more gestation plus number of live
births during the same interval
X 1,000
Number of fetal deaths 20 weeks or
more gestation plus number of
neonatal deaths (28 days or less in
age) during a given interval
Perinatal
Mortality
Rate =
Mortality Measures Related
to Natality (cont.)
26. Epidemiology (Schneider)
Mortality Measures Related
to Natality (cont.)
Estimates events immediately after birth, primarily
congenital malformations, prematurity and low birth weight
Number of live births during
the same interval
X 1,000
Number of deaths of neonates
(28 days or less) in a given
intervalNeonatal Mortality
Rate =
27. Epidemiology (Schneider)
Mortality Measures Related
to Natality (cont.)
Used for international comparisons; high rates indicate
unmet public health needs and poor socioeconomic and
environmental conditions
Number of live births during
the same interval
X 1,000
Number of deaths under 1
year during a given intervalInfant Mortality
Rate =
28. Epidemiology (Schneider)
Mortality Measures Related
to Natality (cont.)
Rates reflect health care access and socioeconomic factors
Number of live births during the
same interval
X 1,000
Number of deaths assigned to
causes related to pregnancy during
a given interval
Maternal
Mortality
Rate =
30. Epidemiology (Schneider)
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
31. Epidemiology (Schneider)
Mortality Outcomes (cont.)
Example: 1980
The larger crude death rate in Florida reflects the
larger population of elderly in that state.
Location Deaths Population
Crude Death Rate
per 1,000
Florida 111,114 10,194,000 10.9
Alaska 1,830 416,000 4.4
32. Epidemiology (Schneider)
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
