Assessing Disease Frequency

Thomas Songer, PhD

Basic Epidemiology

South Asian CardiovascularResearch Methodology Workshop

Why should we be concerned with

monitoring disease(s)?

Why should we be concerned with

monitoring disease(s)?

Disease Control and Prevention

Good monitoring does not necessarily ensure the making of right decisions, but it reduces the risk of wrong ones.

Languimer, 1963

There are several decisions regarding disease monitoring

• What level of resources should be allocated to disease monitoring?

• What outcomes do we want to achieve?

• What benefits are obtained from these items and to whom do the benefits accrue?

• Can the counting methods be readily accepted into the community?

Approaches Towards Monitoring Disease and Injury

Death Certificates

Population Surveys

Surveillance

Registries

Screening

Surveillance:

Systematic, regular ascertainment of incidence using methods distinguished by their practicality, uniformity, and frequently their rapidity, rather than by complete accuracy.

Last, 1990

Types of Surveillance

•Active

•Passive

Active Surveillance• the collection of data on a disease

by regular outreach. Designated medical personnel are called at regular intervals to collect information on the new cases of disease.

monitoring domestic violence in emergency departments

Active Surveillance

Health Dept.

Passive Surveillance

• data generated without contact by the agency carrying out the surveillance. Reportable diseases fall under this type of surveillance.

Passive Surveillance

Health Dept.

Comparison of Disease Counting Approaches

RegistriesPopulationScreening

CommunicableDisease

Surveillance

Source AcademiaAcademiaHealth Depts

Speed Slow Slow Fast

Cost/Case High High Low

Ascertain-ment

>90 % 65-75% Low andVariable

So you have identified the frequency of disease in a given area….

What do you do with it?

Epidemiology is a Science of Rates

• death rates

• disability rates

• hospitalization rates

• incidence rates

• prevalence rates

Rates

• Rates are the basic tool of epidemiologic practice

• Why are rates important?

• because they provide more complete information to describe or assess the impact of disease in a community or population

• Rate: a measure of the occurrence of a health event in a population group at a specified time period

Number of eventsin time period

Number at riskfor the event

numerator

denominator:

Why are rates useful?

• Can help to identify groups with an elevated risk of disease

– can target interventions to these groups– these groups can be studied to identify

risk factors

Page, Cole 1995

Rates

• Relate health events to a population base

• This provides a basis for making valid comparisons of health events by considering the number at risk in each population

Page, Cole 1995

Injury Deaths, Australia, 1992

0

100

200

300

400

500

600

700

10-14yrs

20-24 30-34 40-44 50-54 60-64 70-74 80-84

num

ber

of d

eath

s

Male Female

Age Group

Harrison, 1995

Injury Death Rates, Australia, 1992

0

50

100

150

200

10-14yrs

20-24 30-34 40-44 50-54 60-64 70-74 80-84

deat

hs p

er 1

00,0

00 p

op.

Male Female

Age Group

Harrison, 1995

• Mortality: is one of the major measures of disease in the population

• information available from death certificates (required by law)

• Death rate:

Number of deathsin time period

Number at riskof dying

=

Three common types of rates

• Crude rates

• Specific rates

• Adjusted rates

Page, Cole 1995

Three common types of rates

• Crude rates– consider the entire population

• Specific rates– consider differences among subgroups of the population

• Adjusted rates– adjust for differences in population composition

Page, Cole 1995

• Crude rates

• Specific rates

Crude death rate =

number of deaths in time period

total population

Age-specific death rate

number of deaths in age group in time period

population in age group=

Adjusted Rates• Use statistical procedures to adjust for

differences in characteristics between populations

• Age is the most frequent factor adjusted for because age is related to both death and disease

• Adjusted rates do not describe actual occurrence, but are hypothetical given certain assumptions

Crude and Age-Adjusted Death RatesUnited States, 1940-1992

0

200

400

600

800

1000

1200

1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992

per

100,

000

popu

lati

on crude death rate

age-adjusted death rate

Morbidity

• any departure from health

• i.e. the extent of illness, injury or disability in a defined population

• morbidity rates are used as indicators of health

• in epidemiology, the main measures of morbidity are incidence and prevalence

Measures of disease frequency

• Incidence rates– crude incidence– cumulative incidence

• Prevalence rates– crude prevalence– period prevalence– point prevalence

Paneth

• Incidence: is one of the major measures of disease in the population

• information available from surveys, registries, or investigations

• Incidence rate:

Number of new casesof disease in population

in time period

Number at risk ofdeveloping disease in

same time period

=

x 1000

Incidence Rate

• The numerator has to come from the population at risk for developing disease

• The denominator may change over time as people develop disease

• The denominator does not include persons with the disease

numerator

denominator

Incidence Rate

• in large studies, the denominator is often the mid-year population

• in small studies, the denominator does not include persons with the disease

numerator

denominator

However, in practice

What is the incidence rate fromOctober 1, 1990 to Sep 30, 1991?

What is the incidence rate fromOctober 1, 1990 to Sep 30, 1991?

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Cumulative incidence

• Number of new cases of disease occurring over a specified period of time in a population at risk (at the beginning of the interval)

Number of new cases of disease identified over a given time

interval

Estimated populationat beginning of interval

=Cumulative

incidencerate

Prevalence

• Prevalence: is another major measure of disease in the population

• information available from surveys, registries, or investigations

Number of existing casesof disease in population

in time period

Population at risk in same time period

=

x 1000

PrevalenceRate

Difference between incidence rates and prevalence rates

• Numerator: New cases occurring during a given time period

• Denominator: Number at risk of developing disease

• Numerator: All cases present (new and existing) during a given time period

• Denominator: Number in population

Incidence Prevalence

Several factors may affect prevalence

• Incidence

• Duration of disease

• Disease treatments

Prevalence rate

Incidence Prevalence

Longer duration Prevalence

Better treatment Prevalence

=Incidence rate x average

duration of disease

Point Prevalence

• Number of individuals in a specified population at risk who have the disease of interest at a given point in time

Number of cases of disease at a given point in time

Estimated populationat the same point in time

=Point

prevalencerate

What is the point prevalence on April 1?

What is the point prevalence on April 1?

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