Measures of Disease Occurrence MH, Emamian. MD, MPH, PhD

Objectives  Define and use: Ratio Proportion Rate  Define and use: Prevalence: Point / Period Incidence: Cumulative incidence / Incidence rate

Importance of denominators  Example: Cases of maternal mortality ( ) : 85 in Tehran 25 in Bushehr  Which city has the worse situation??

Importance of denominators  Example: Cases of maternal mortality ( ) : 85 in Tehran 25 in Bushehr  Which city has the worse situation?? Tehran: 85 / ≈ 10 /100,000 Bushehr: 25 / ≈ 35 /100,000 Bushehr has 3.5 times more maternal mortality

What, who is in the denominator ? ??? Ratio Proportion Rate

= 5 / 2 = 2.5 / 1 The quotient of 2 numbers Numerator NOT necessarily INCLUDED in the denominator Allows to compare quantities of different nature Ratio 6

Ratio, Examples  # population per doctor 8500 person/10 doctors R = 850 person for 1 doctor  # participants per facilitator  Sex ratio:Male / Female Female / Male  Odds ratio  Rate ratio  Prevalence ratio 7

2 --- = 0.5 = 50% 4 Proportion The quotient of 2 numbers Numerator NECESSARELY INCLUDED in the denominator Quantities have to be of same nature Proportion always ranges between 0 and 1 Percentage = proportion x 100

Proportion, Examples  Proportion of Kermani students in class: 80 trainees, 4 from Kerman Proportion of Kermani students = 0.05 Percentage of Kermani students = 5 % 7 Oct 079

Rate The quotient of 2 numbers Speed of occurrence of an event over time Observed in 1998 Numerator: - number EVENTS observed for a given time 7 Oct 0710

Rate The quotient of 2 numbers Speed of occurrence of an event over time Observed in 1998 Numerator: - number EVENTS observed for a given time Denominator: - population in which events occur (pop. at risk) - includes time 7 Oct = ---- = 0.02 /year 100

Rate, Example  Mortality rate of tetanus in France in 1995 Tetanus deaths: 17 Population in 1995: 58 million Mortality rate = 0.029/100,000/year  Rate may be expressed in any power of ; 1,000; 10,000; 100,000

Measures of disease occurence  Epidemiology : « Study of the occurrence of illness »  Two types of measures : Prevalence Measures population disease status Incidence Assess frequency of disease onset  Cumulative incidence  Incidence rate

Prevalence Number of cases of disease at a specific time Population exposed at that time  Proportion of a population affected by a disease at a given time - Ex. Number of people living with HIV (2006) Population 69,515,000 Cases 66,000 Prevalence 0.09%

Prevalence  Point  Prevalence of a disease at a point in time  Period  Proportion of people who have had the disease at any time during a certain period

ConceptDefinitionFormula Point prevalence Proportion of people in a population having disease at a particular point in time No of subjects having disease at a particular point in time Total no of subjects in the population Period prevalence Proportion of people in a population having disease over a period of time No of subjects with disease at the start of the period + no of subjects developing disease over the time period Total no of subjects in the population

Cumulative Incidence (CI) Number of NEW cases of disease during a period Population exposed during this period = Incidence Proportion Ex. Maternal Mortality in : Population 22,000,000 New cases 300 Cumulative incidence1.4/100,000 per year

Hypothetical example D (1) D (5) (6) D (3) Cumulative incidence = 3/5 = 0.6 (Incidence Proportion )

Incidence Rate Number of NEW cases of disease during a period Total person-time of observation Instantaneous concept (like speed) Denominator: - is a measure of time - the sum of each individual’s time at risk and free from disease

Example D (1) D (5) (6) D (3) Incidence Proportion = 3/5 = 0.6 Incidence rate = 3/21 = 0.14 month -1 Incidence rate (PY) = /12+3/12+5/12+6/12+6/12 =3 /1.75 = 1.71 yr -1

Deaths, Cured, Lost... Duration Prevalence Incidence Incidence, Prevalence & Disease Duration

Prevalence & Incidence Point Prevalence ≈ Incidence × duration 7 Oct 0722

7 Oct 0723 Iran, Ardebil