MEASURES IN MEASURES IN EPIDEMIOLOGYEPIDEMIOLOGY
Dwi Agustian, dr., MPHDwi Agustian, dr., MPHDept. Epidemiology & BiostatisticsDept. Epidemiology & Biostatistics
FACULTY OF MEDICINE – FACULTY OF MEDICINE – UNIVERSITAS PADJADJARANUNIVERSITAS PADJADJARAN
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.
General FormulaGeneral Formula
Numerator(x)
Denominator(y) Measure =
The basic formula is as follows:
RateRate
Number of cases or events occurring during a given time period
Population at Risk during the same time period
Rate =
The basic formula for a rate is as follows:
Use of Ratios, Proportions, and RatesUse of Ratios, Proportions, and RatesConditionCondition RatiosRatios ProportionsProportions RatesRates
MorbidityMorbidity
(Disease)(Disease)
Risk Ratio Risk Ratio
(Relative Risk)(Relative Risk)
Rate RatioRate Ratio
Odds RatioOdds Ratio
Attributable Attributable proportionproportion
Point PrevalencePoint Prevalence
Incidence rateIncidence rate
Attack rateAttack rate
Secondary attack rateSecondary attack rate
Person-time ratePerson-time rate
Period PrevalencePeriod Prevalence
MortalityMortality
(Death)(Death)
Death-to-case ratioDeath-to-case ratio
Maternal Mortality rateMaternal Mortality rate
Proportionate mortality Proportionate mortality raterate
Postneonatal mortality Postneonatal mortality raterate
Proportionate Proportionate mortalitymortality
Case-fatality rateCase-fatality rate
Crude mortality rateCrude mortality rate
Cause-specificCause-specific
mortality ratemortality rate
Age-specific mortality Age-specific mortality raterate
Race-specific mortality Race-specific mortality raterate
Age adjusted mortality Age adjusted mortality raterate
NatalityNatality
(Birth)(Birth)
Low birth weight Low birth weight ratioratio
Crude birth rateCrude birth rate
Crude fertility rateCrude fertility rate
Crude rate of natural Crude rate of natural increaseincrease
Risk RatioRisk Ratio
Risk for Group of primary interestRR =
The formula for Risk Ratio is :
Risk for Comparison Group
Rate RatioRate Ratio
Rate for Group of primary interestRR =
The formula for Rate Ratio is :
Rate for Comparison Group
Odds RatioOdds Ratio
adOR =
The formula for Odds Ratio is :
bc
++ --
++ aa bb
-- cc dd
Disease/Outcome
Exposure/
Cause
Attributable ProportionAttributable Proportion
Risk for exposed group – Risk for unexposed group
Risk for exposed groupAR =
The formula for attributable proportion is :
X 100%
Person-time RateRate
# cases during observation period
Time each person observed, Totaled for all person
PtR =
The formula for person time rate is :
X 10 n
Incidence RateIncidence Rate
# new cases of a specifieddisease reported during a given time interval
Average population during time intervalIR =
The formula for incidence rate is :
Attack RateAttack Rate
# new cases of a specified diseases reported during an epidemic period
Population at start of The epidemic periodAR =
The formula for attack rate is :
Secondary Attack RateSecondary Attack Rate
# new cases of a specified
diseases among contacts of known cases
Size of contact Population at riskSAR =
The formula for secondary attack rate is :
Point PrevalencePoint Prevalence
# current cases, new and old, of a specified disease at a given point in time
Estimated population at the same point in timePoP =
The formula for point prevalence is :
Period PrevalencePeriod Prevalence
# current cases, new and old, of a specified disease identified over a given time interval
Estimated population at mid-intervalPeP =
The formula for period prevalence is :
Frequently Used Measures of MorbidityFrequently Used Measures of Morbidity
Measure Numerator (x) Denominator (y) Expressed perNumber at Risk
(10n)
Incidence Rate
Attack Rate
SecondaryAttack Rate
PointPrevalence
PeriodPrevalence
# new cases of a specified
disease reported during a given time interval
# new cases of a specified diseases reported during an epidemic period
# new cases of a specified
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 contactPopulation at risk
Estimated population at the same point in time
Estimated population at mid-interval
Varies :10n wheren = 2,3,4,5,6varies10n wheren = 2,3,4,5,6
varies10n wheren = 2,3,4,5,6varies10n wheren = 2,3,4,5,6
varies10n wheren = 2,3,4,5,6