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

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.
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.
Last, 1990

8. Types of Surveillance
Active
Passive

9. monitoring domestic violence in emergency departments
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.
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.
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.
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.

12. Passive Surveillance
Health Dept.

13. 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
>90 %
65-75%
Low and
-ment
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
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.

16. 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. numerator denominator : Number of events in time period Number at risk
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
numerator :
denominator
Number at risk
for the event

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
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.
Harrison, 1995

21. Injury Death Rates, Australia, 1992
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.
Harrison, 1995

22. = Number of deaths in time period Number at risk of dying Death rate:
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 = number of deaths in time period
Crude death rate =
total population
number of deaths in age
group in time period
Age-specific
death rate =
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
crude death rate
per 100,000 population
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. of disease in population
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. numerator denominator 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
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.

32. numerator denominator Incidence Rate 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
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.

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
Cumulative
incidence
rate =
Estimated population
at beginning of interval

36. Prevalence

37. Number of existing cases of disease in population
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
Prevalence
Rate =
Population at risk
in same time period
x 1000

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

39. Several factors may affect prevalence
Incidence
Duration of disease
Disease treatments

40. Incidence rate x average
Prevalence rate
Incidence rate x average
duration of disease =

41. Number of cases of disease at the same point in time
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
Point
prevalence
rate =
Estimated population
at the same point in time

42. What is the point prevalence on April 1?

43. What is the point prevalence on April 1?7
7 / 18