Measures of Disease Measures of Disease OccurrenceOccurrence

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 (1380-83): 85 in Tehran 25 in Bushehr

Which city has the worse situation??

Importance of denominators

Example: Cases of maternal mortality (1380-83): 85 in Tehran 25 in Bushehr

Which city has the worse situation??

Tehran: 85 / 859866 ≈ 10 /100,000

Bushehr: 25 / 72636 ≈ 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

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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

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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 %

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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

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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

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2= ---- = 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 10 100; 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 69,515,000Cases 66,000Prevalence 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

Concept Definition Formula

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 22,000,000New cases 300Cumulative incidence 1.4/100,000 per year

Hypothetical example

0

1

2

3

4

5

0 1 2 3 4 5 6

Follow-up Time (Months)

D (1)

D (5)

(6)

(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 observationInstantaneous concept (like speed)

Denominator:- is a measure of time - the sum of each individual’s time at risk and free from disease

Example

0

1

2

3

4

5

0 1 2 3 4 5 6

Follow-up Time (Months)

D (1)

D (5)

(6)

(6)

D (3)

Incidence Proportion =3/5 = 0.6

Incidence rate =3/21 = 0.14 month-1

Incidence rate (PY) =3

---------------------------------1/12+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

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7 Oct 07 23Iran, Ardebil