This document discusses methods for assessing disease frequency, including calculating rates. It describes the differences between crude rates, specific rates, and adjusted rates. It also discusses the differences between measures of disease occurrence such as incidence rates, cumulative incidence rates, and prevalence rates. Incidence rates measure new cases over time, while prevalence rates measure all existing cases at a point in time. Calculating accurate rates allows for meaningful comparisons of disease burden and targeting of interventions.

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)?

3. Disease Control and Prevention

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

8. Types of Surveillance
•Active
•Passive

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

10. Active Surveillance
Health Dept.

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

12. Passive Surveillance
Health Dept.

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
number
of
deaths
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
deaths
per
100,000
pop.
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
per
100,000
population
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

29. Measures of disease frequency
• Incidence rates
–crude incidence
–cumulative incidence
• Prevalence rates
–crude prevalence
–period prevalence
–point prevalence
Paneth

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?
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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

36. Prevalence

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

42. What is the point prevalence on April 1?

43. What is the point prevalence on April 1?
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