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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)?
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
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
Passive Surveillance
• data generated without contact by the agency carrying out the surveillance. Reportable diseases fall under this type of surveillance.
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
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– 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
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: 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
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