Measuring disease and death frequency

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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, proportion
2. Prevalence
3. Incidence
4. Relation between prevalence and incidence
5. 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 cases
• Native Americans 19 (8 of cases)
• Others 228 (92 of cases)
• Can you compare these two groups with this
  information?
• How can this information be used?
• Who can use this information?

Introduction
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Count, divide and compare
2. Divide the number of cases by the population
New hepatitis A cases New hepatitis A cases New hepatitis A cases
Year Population
Native Americans 19 1996 1,697
Others 228 1996 96,576
Native Americans 19/1,697 Others 228/96,579
Introduction
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Count, divide and compare
3. Compare indicators
Native Americans 1,112 per 100,000 Others
236/100,000/ yearRates among Native Americans
are higher
Introduction
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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

5 / 2 2.5/1
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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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

2 / 4 0.550
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

Observed in 2004
Introduction
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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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The dynamic of incidence and prevalence
New cases
Incidence
Prevalence
Death Cure
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 populations
  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
• Dont 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