1. MEASURES IN EPIDEMIOLOGY
Dwi Agustian, dr., MPH
Dept. Epidemiology & Biostatistics
FACULTY OF MEDICINE – UNIVERSITAS PADJADJARAN

2. Objectives
After studying this lesson and answering the questions in the exercises, a student will be able to do the following:
• Calculate* and interpret the following statistical measures:
— ratios
— proportions
— incidence rates, including attack rate
— prevalence
— risk ratio, rate ratio, odds ratio, attributable proportion.

3. Ratio & Proportion
Female
All
Female
Male
Ratio
Proportion

4. General Formula The basic formula is as follows: Numerator(x)
Measure =
Denominator(y)

5. Rate The basic formula for a rate is as follows:
Number of cases or events occurring
during a given time period
Rate =
Population at Risk during the same time period

6. Use of Ratios, Proportions, and Rates
Condition
Ratios
Proportions
Rates
Morbidity
(Disease)
Risk Ratio
(Relative Risk)
Rate Ratio
Odds Ratio
Attributable proportion
Point Prevalence
Incidence rate
Attack rate
Secondary attack rate
Person-time rate
Period Prevalence
Mortality
(Death)
Death-to-case ratio
Maternal Mortality rate
Proportionate mortality rate
Postneonatal mortality rate
Proportionate mortality
Case-fatality rate
Crude mortality rate
Cause-specific
mortality rate
Age-specific mortality rate
Race-specific mortality rate
Age adjusted mortality rate
Natality
(Birth)
Low birth weight ratio
Crude birth rate
Crude fertility rate
Crude rate of natural increase

7. Risk Ratio The formula for Risk Ratio is :
Risk for Group of primary interest
RR =
Risk for Comparison Group

8. Rate Ratio The formula for Rate Ratio is :
Rate for Group of primary interest
RR =
Rate for Comparison Group

9. Odds Ratio The formula for Odds Ratio is : ad OR = bc + - a b c d
Disease/Outcome ad + - a b c d bc
OR =
Exposure/Cause

10. Attributable Proportion
The formula for attributable proportion is :
Risk for exposed group – Risk for unexposed group
AR =
X 100%
Risk for exposed group

11. Person-time Rate The formula for person time rate is :
# cases during observation period
PtR =
X 10 n
Time each person observed, Totaled for all person

12. Incidence Rate The formula for incidence rate is : IR =
# new cases of a specified
disease reported during a given time interval
IR =
Average population during time interval

13. The formula for attack rate is :
# new cases of a specified diseases reported during an epidemic period
AR =
Population at start of The epidemic period

14. Secondary Attack Rate The formula for secondary attack rate is : SAR =
# new cases of a specified
diseases among contacts of known cases
SAR =
Size of contact Population at risk

16. Point Prevalence The formula for point prevalence is : PoP =
# current cases, new and old, of a specified disease at a given point in time
PoP =
Estimated population at the same point in time

17. Period Prevalence The formula for period prevalence is : PeP =
# current cases, new and old, of a specified disease identified over a given time interval
PeP =
Estimated population at mid-interval

18. Frequently Used Measures of Morbidity
Numerator (x)
Denominator (y)
Expressed per
Number at Risk (10n)
Incidence Rate
Attack Rate
Secondary
Point
Prevalence
Period
# new cases of a specified
disease reported during a given time interval
# new cases of a specified diseases reported during an epidemic period
diseases among contacts of known cases
# current cases, new and old, of a specified disease at a given point in time
# current cases, new and old, of a specified disease identified over a given time interval
Average population during time interval
Population at start of
The epidemic period
Size of contact
Population at risk
Estimated population at the same point in time
Estimated population at mid-interval
Varies :
10n where
n = 2,3,4,5,6
varies

28. Thank You