Assessing disease frequency
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Transcript of Assessing disease frequency
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Assessing Disease Frequency
Thomas Songer, PhD
Basic Epidemiology
South Asian CardiovascularResearch Methodology Workshop
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Why should we be concerned with
monitoring disease(s)?
Why should we be concerned with
monitoring disease(s)?
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Disease Control and Prevention
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Good monitoring does not necessarily ensure the making of right decisions, but it reduces the risk of wrong ones.
Languimer, 1963
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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?
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Approaches Towards Monitoring Disease and Injury
Death Certificates
Population Surveys
Surveillance
Registries
Screening
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Surveillance:
Systematic, regular ascertainment of incidence using methods distinguished by their practicality, uniformity, and frequently their rapidity, rather than by complete accuracy.
Last, 1990
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Types of Surveillance
•Active
•Passive
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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
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Active Surveillance
Health Dept.
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Passive Surveillance
• data generated without contact by the agency carrying out the surveillance. Reportable diseases fall under this type of surveillance.
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Passive Surveillance
Health Dept.
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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
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So you have identified the frequency of disease in a given area….
What do you do with it?
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Epidemiology is a Science of Rates
• death rates
• disability rates
• hospitalization rates
• incidence rates
• prevalence rates
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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
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• 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:
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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
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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
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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
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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
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• 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
=
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Three common types of rates
• Crude rates
• Specific rates
• Adjusted rates
Page, Cole 1995
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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
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• 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=
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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
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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
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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
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Measures of disease frequency
• Incidence rates– crude incidence– cumulative incidence
• Prevalence rates– crude prevalence– period prevalence– point prevalence
Paneth
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• 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
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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
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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
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What is the incidence rate fromOctober 1, 1990 to Sep 30, 1991?
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What is the incidence rate fromOctober 1, 1990 to Sep 30, 1991?
4
4 / 14
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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
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Prevalence
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• 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
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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
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Several factors may affect prevalence
• Incidence
• Duration of disease
• Disease treatments
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Prevalence rate
Incidence Prevalence
Longer duration Prevalence
Better treatment Prevalence
=Incidence rate x average
duration of disease
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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
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What is the point prevalence on April 1?
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What is the point prevalence on April 1?
7
7 / 18