Measuring disease and death frequency Integrated Disease Surveillance Programme (IDSP) district surveillance officers (DSO) course.

Measuring disease and death frequency

Integrated Disease Surveillance Programme (IDSP) district surveillance

officers (DSO) course

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Outline of the session

1. Ratio, rate, proportion2. Prevalence3. Incidence4. Relation between prevalence and

incidence5. Mortality

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Count, divide and compare:

1.Question during an outbreak of hepatitis A in Sioux City, IA, USA: Are native Americans at higher risk?

Number of new hepatitis A casesNative Americans: 19 (8% of cases)

Others: 228 (92% of cases)

a. Can you compare these two groups with this information?

b. How can this information be used?c. Who can use this information?

Introduction

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New hepatitis A cases

# Year Population

Native Americans

19 1996 1,697

Others 228 1996 96,576

2. Divide the number of cases by the population

Native Americans: 19/1,697

Others: 228/96,579

Count, divide and compare:

Introduction

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Native Americans: 1,112 per 100,000

Others: 236/100,000/ year

Rates among Native Americans are higher

3. Compare indicators

Count, divide and compare:

Introduction

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= 5 / 2 = 2.5/1

A ratio places in relation two quantities that may be

unrelated• The quotient of two numbers• Numerator NOT necessarily INCLUDED in the denominator

• Allows comparing quantities of different nature: Female to male ratio

Introduction

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Examples of ratio

• Number of doctor per beds 1 doctor for 85 beds

• Number of participants per facilitator

• Sex ratio: Females / Males• Ratio of white blood cells to red blood cells 1/600. What does it tell?

• Number of children with scabies / number of children with malnutrition Does it make sense?

Introduction

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2 / 4 = 0.5=50%

A proportion measures a subset of a total quantity

• The quotient of two numbers• Numerator NECESSARILY INCLUDED in the denominator

• Quantities have to be of the same nature

• Proportion always ranges between 0 and 1

• Percentage = proportion x 100

Introduction

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Example of proportions

• The proportion of children with scabies in a village

• Tuberculosis cases in a district: 400 males, 200 females

• Question What is the proportion of males among all cases? What is the proportion of females cases among all cases?

• Note: All proportions are ratios Is the converse true?

Introduction

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A rate measures the speed of occurrence of health events

• The quotient of two numbers• Defined duration of observation• Numerator

Number of EVENTS observed for a given time

• Denominator (includes time) Population at risk in which the events occur 2

----- = 0.02 / year 100

Introduction

Observed in 2004

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Example of rate

• Mortality rate of tetanus in country X in 1995 Tetanus deaths: 17 Population in 1995: 58 million Mortality rate = 0.03/100,000/year

• Rate may be expressed in any power of 10 100; 1,000; 10,00; 100,000

Introduction

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Prevalence – (P)

• Number of existing cases (old and new) in a defined population at a specified point of time

Number pf people with disease at a specified time P =

---------------------------------------------------- x 10n

Population at risk at the specified time

• In some studies the total population is used as an approximation if data on population at risk is not available

Prevalence

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Source and type of prevalence data

• Surveys generate prevalence data• Prevalence data are expressed as proportions Number affected / Number surveyed The numerator is included in the denominator• The affected are only identified among the surveyed

Prevalence

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Example of point prevalence

• 150 children in a school• Screening for visual acuity at a given time

• 15 children require glasses• Prevalence of refractory errors

15 / 150 = 10%

Prevalence

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Factors influencing prevalence

• Number of new cases• Duration of the illness

If the disease is short, the prevalence is reduced• The prevalence of sudden infant death = 0(At a given instant, nobody has sudden infant death because the disease has no duration)

If the disease is long, the prevalence is increased• Rare lifelong disease can accumulate to build up a large prevalence

Prevalence

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Causes of increase of prevalence

• Long duration Low cure rate Low case fatality

• Increase in new cases• Immigration of patients• Improved detection• Emigration of healthy people

Prevalence

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Causes of decrease of prevalence

• Shorter duration High cure rate High case fatality

• Decrease in new cases • Emigration of patients • Improved cure rate• Immigration of healthy people

Conclusion: Changes in prevalence may have many causes and may be difficult to interpret

Prevalence

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Uses of prevalence data

• Assessing health care needs • Planning health services services• Measure occurrence of conditions with gradual onset

• Study chronic diseases

Prevalence

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Incidence – (I)

• Number of new cases in a given period in a specified population Time, (i.e., day, month, year) must be specified

• Measures the rapidity with which new cases are occurring in a population

• Not influenced by the duration of the disease

Incidence

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Cumulated incidence - (CI)

Number of new cases CI =

-------------------------------------------------- x 10n

Population at risk at the beginning

• Also known as: Attack rate

• Assumes that the entire population at risk at the beginning was followed-up for the time period of observation

Incidence

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Source and type of incidence data

• Surveillance generate incidence data

• Incidence data are expressed as rates Number affected / Population / time Dynamic measure (speed)

Prevalence

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Uses of incidence data

• Describe trends in diseases • Evaluate impact of prevention programmes

Incidence

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Prevalence

Incidence

Death

Cure

New cases

The dynamic of incidence and prevalence

Incidence and prevalence

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The relation between prevalence

and incidence• Prevalence depends on

Incidence (I) Duration of the disease (D)

P = I x D • Change in prevalence from one time period to another may be the result of changes in incidence rates, changes in the duration of disease, or both

Incidence and prevalence

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Patterns of incidence and prevalence

• High prevalence and low incidence e.g., Diabetes Mellitus

• Low prevalence and high incidence e.g., Common cold

Incidence and prevalence

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Evolution of HIV prevalence in a country scaling up public

health efforts• Increase in HIV prevention

Reduction in incidence (Difficult to measure)

• Increase in HIV AIDS care and support (treatment) Increase in disease duration (reduced mortality) Increase in prevalence (Easier to measure)

• Incidence measures the impact of prevention efforts

• Prevalence may be used to plan care and support

• The immediate consequence of the plan may be an increased prevalence

Incidence and prevalence

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Crude mortality rate - (CMR)

Number of deaths in a specified period CMR =

------------------------------------------------ x 10n Average total

population

• Does not take into account factors such as age, sex, race, socio economic status, etc.

• Provides information on trends in a population’s health status

Deaths

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Disease specific mortality rate - (SMR)

Number of deaths from a disease in a specified period SMR =

---------------------------------------------------------------- x 10n

Average total population

• Reflect the impact of a disease on a population in terms of death

• Should not be confused with case fatality

Deaths

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Case fatality ratio of a given disease

• Divide Number of deaths from the disease Number of cases of the disease

• Example: Measles outbreak 3 deaths 145 cases Case fatality ratio: 2.1%

• Don’t mix up with disease-specific mortality!

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Take home messages

• Tell apart ratio, proportion and rates

• Prevalence is a static measure taken at a point in time

• Incidence is a dynamic measure taken over a certain time

• Mortality is calculated using population denominators to reflect burden while case fatality is calculated using cases as denominators to reflect severity