Transcript South Asian Cardiovascular Research Methodology Workshop Basic Epidemiology Assessing Disease Frequency Thomas Songer, PhD.

South Asian Cardiovascular
Research Methodology Workshop
Basic Epidemiology
Assessing Disease Frequency
Thomas Songer, PhD
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
Population Communicable
Registries Screening
Disease
Surveillance
Source
Academia Academia
Health Depts
Speed
Slow
Slow
Fast
Cost/Case
High
High
Low
Ascertain
-ment
>90 %
65-75%
Low and
Variable
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
numerator
:
denominator
Number of events
in time period
Number at risk
for the event
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
number of deaths
Male
700
600
500
400
300
200
100
0
10-14
yrs
20-24
30-34
Female
40-44
50-54
60-64
70-74
80-84
Age Group
Harrison, 1995
Injury Death Rates, Australia, 1992
deaths per 100,000 pop.
Male
Female
200
150
100
50
0
10-14
yrs
20-24
30-34
40-44
50-54
60-64
70-74
80-84
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 deaths
in time period
=
Number at risk
of 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
Crude death rate =
number of deaths
in time period
total population
• Specific rates
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 Rates
United States, 1940-1992
per 100,000 population
1200
1000
crude death rate
800
600
400
age-adjusted death rate
200
0
1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992
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
Number of new cases
of disease in population
• Incidence rate:
in time period
=
x 1000
Number at risk of
developing disease in
same time period
Incidence Rate
numerator
denominator
• 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
Incidence Rate
numerator
denominator
However, in practice
• in large studies, the denominator is
often the mid-year population
• in small studies, the denominator does
not include persons with the disease
What is the incidence rate from
October 1, 1990 to Sep 30, 1991?
What is the incidence rate from
October 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
Cumulative
interval
incidence =
Estimated population
rate
at beginning of interval
Prevalence
• Prevalence: is another major
measure of disease in the population
• information available from surveys,
registries, or investigations
Prevalence
Rate
x 1000
=
Number of existing cases
of disease in population
in time period
Population at risk
in same time period
Difference between incidence
rates and prevalence rates
Incidence
• Numerator: New
cases occurring
during a given time
period
• Denominator:
Number at risk of
developing disease
Prevalence
• Numerator: All cases
present (new and
existing) during a
given time period
• Denominator:
Number in
population
Several factors may affect
prevalence
• Incidence
• Duration of disease
• Disease treatments
Prevalence
rate
Incidence rate x average
= duration of disease
Incidence
Prevalence
Longer duration
Prevalence
Better treatment
Prevalence
Point Prevalence
• Number of individuals in a specified
population at risk who have the disease
of interest at a given point in time
Point
prevalence
rate
=
Number of cases of disease
at a given point in time
Estimated population
at the same point in time
What is the point prevalence on April 1?
What is the point prevalence on April 1?
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