Let’s spend some more time talking about each of the major systems for identifying injuries. The primary means to identify injuries is to establish a surveillance system. What is a surveillance system? John Last has defined surveillance as the regular assessment of disease or injuries, with a common method, and often an approach that is simplistic rather than complex. One of the key words here is regular assessment. Injury monitoring, such as that with death certificates, is set up to evaluate injuries over time. With common methods, this allows a researcher to examine changes over time in these events.
Active surveillance involves the regular monitoring of surveillance sites by designated persons. These persons often call up a site to gather information on injury events that happened in the previous month or week. One example of this process would be the surveillance of injuries from domestic violence in emergency departments. Research personnel would contact emergency departments on a regular interval to identify injury events from domestic violence. As no data sources routinely identify domestic violence injuries, this type of active surveillance is necessary to identify the incidence of domestic violence events.
More commonly surveillance systems are passive. By definition, researchers or health department personnel do not go out into the community to find cases. Rather, they develop instruments that persons in the community have to send into them (e.g. death certificates) as a means of identifying events. Reportable diseases such as AIDS and malaria, etc. are monitored in this fashion. Laboratories, physicians, and hospitals have to report these events to the health department when they identify a case at their institution. In some states, spinal cord injuries are a reportable condition and are followed with passive surveillance.
Epidemiology is a science based upon rates. Incidence and prevalence rates provide information on the importance of a disease or injury in a defined population. Understanding how many injuries occur (the absolute number) provides only one part of the puzzle; the numerator. The second piece is the population in which they occur; the denominator. As we will see, the denominator is a very important factor in defining the significance of a problem. Effective injury control is based upon an understanding of injury rates; i.e. how many injuries occur within a given population.
In this slide, we see the number of injury deaths in Australia by age and gender. The graph suggests that injury deaths are particularly significant in young adults and men.
In this slide, we see the injury death rates in Australia by age and gender. The message of the graph has changed from that previously. Here we see that injuries are significant in both the young and the very old (75+ years). The previous slide considered only the “numerator”; the number of injury deaths. It’s message is different from this slide, which included a denominator (the population in the respective age and gender categories). Thus, numerator and denominator data are both important to defining the magnitude of the problem.
Epidemiology is a science based upon rates. Incidence and prevalence rates provide information on the importance of a disease or injury in a defined population. Understanding how many injuries occur (the absolute number) provides only one part of the puzzle; the numerator. The second piece is the population in which they occur; the denominator. As we will see, the denominator is a very important factor in defining the significance of a problem. Effective injury control is based upon an understanding of injury rates; i.e. how many injuries occur within a given population.
Epidemiology is a science based upon rates. Incidence and prevalence rates provide information on the importance of a disease or injury in a defined population. Understanding how many injuries occur (the absolute number) provides only one part of the puzzle; the numerator. The second piece is the population in which they occur; the denominator. As we will see, the denominator is a very important factor in defining the significance of a problem. Effective injury control is based upon an understanding of injury rates; i.e. how many injuries occur within a given population.
Transcript
1.
Assessing Disease Frequency
Thomas Songer, PhD
Basic Epidemiology
South Asian Cardiovascular
Research Methodology Workshop
2.
Why should we be
concerned with
monitoring disease(s)?
4.
Good monitoring does not necessarily
ensure the making of right decisions,
but it reduces the risk of wrong ones.
Languimer, 1963
5.
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?
6.
Approaches Towards Monitoring
Disease and Injury
Death Certificates
Population Surveys
Surveillance
Registries
Screening
7.
Surveillance:
Systematic, regular ascertainment
of incidence using methods
distinguished by their practicality,
uniformity, and frequently their
rapidity, rather than by complete
accuracy.
Last, 1990
9.
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
11.
Passive Surveillance
• data generated without contact by
the agency carrying out the
surveillance. Reportable diseases
fall under this type of surveillance.
13.
Comparison of Disease Counting
Approaches
Registries
Population
Screening
Communicable
Disease
Surveillance
Source Academia Academia Health Depts
Speed Slow Slow Fast
Cost/Case High High Low
Ascertain
-ment
>90 % 65-75% Low and
Variable
14.
So you have identified the frequency
of disease in a given area….
What do you do with it?
15.
Epidemiology is a Science of Rates
• death rates
• disability rates
• hospitalization rates
• incidence rates
• prevalence rates
16.
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
17.
• Rate: a measure of the occurrence
of a health event in a population
group at a specified time period
Number of events
in time period
Number at risk
for the event
numerator
denominator
:
18.
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
19.
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
20.
Injury Deaths, Australia, 1992
0
100
200
300
400
500
600
700
10-14
yrs
20-24 30-34 40-44 50-54 60-64 70-74 80-84
numberofdeaths
Male Female
Age Group
Harrison, 1995
21.
Injury Death Rates, Australia, 1992
0
50
100
150
200
10-14
yrs
20-24 30-34 40-44 50-54 60-64 70-74 80-84
deathsper100,000pop.
Male Female
Age Group
Harrison, 1995
22.
• 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
=
23.
Three common types of rates
• Crude rates
• Specific rates
• Adjusted rates
Page, Cole 1995
24.
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
25.
• 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
=
26.
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
27.
Crude and Age-Adjusted Death Rates
United States, 1940-1992
0
200
400
600
800
1000
1200
1940 1944 1948 1952 1956 1960 1964 1968 1972 1976 1980 1984 1988 1992
per100,000population
crude death rate
age-adjusted death rate
28.
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
30.
• Incidence: is one of the major
measures of disease in the population
• information available from surveys,
registries, or investigations
• Incidence rate:
Number of new cases
of disease in population
in time period
Number at risk of
developing disease in
same time period
=
x 1000
31.
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
32.
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
33.
What is the incidence rate from
October 1, 1990 to Sep 30, 1991?
34.
What is the incidence rate from
October 1, 1990 to Sep 30, 1991?
4
4 / 14
35.
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 population
at beginning of interval
=
Cumulative
incidence
rate
37.
• Prevalence: is another major
measure of disease in the population
• information available from surveys,
registries, or investigations
Number of existing cases
of disease in population
in time period
Population at risk
in same time period
=
x 1000
Prevalence
Rate
38.
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
39.
Several factors may affect
prevalence
• Incidence
• Duration of disease
• Disease treatments
40.
Prevalence
rate
Incidence Prevalence
Longer duration Prevalence
Better treatment Prevalence
=
Incidence rate x average
duration of disease
41.
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 population
at the same point in time
=
Point
prevalence
rate
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