Measuring disease and death frequency

Integrated Disease Surveillance Programme (IDSP) district surveillance

officers (DSO) course

2

Outline of the session

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

incidence5. Mortality

3

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

4

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

5

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

6

= 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

7

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

8

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

9

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

10

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

11

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

12

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

13

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

14

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

15

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

16

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

17

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

18

Uses of prevalence data

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

• Study chronic diseases

Prevalence

19

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

20

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

21

Source and type of incidence data

• Surveillance generate incidence data

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

Prevalence

22

Uses of incidence data

• Describe trends in diseases • Evaluate impact of prevention programmes

Incidence

23

Prevalence

Incidence

Death

Cure

New cases

The dynamic of incidence and prevalence

Incidence and prevalence

24

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

25

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

26

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

27

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

28

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

29

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!

30

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