MEASURES IN EPIDEMIOLOGY Dwi Agustian, dr., MPH Dept. Epidemiology & Biostatistics FACULTY OF MEDICINE – UNIVERSITAS 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 & Proportion Female All Female Male Ratio Proportion

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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