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.

Ratio & ProportionRatio & Proportion

FemaleFemale

AllAll

FemaleFemale

MaleMale

Ratio

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

Thank You