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1. Incidence and Prevalence
Measures of Disease Frequency
Ram Chandra Sharma
Group-218

2. Objectives
 Importance of Measures of Disease
Frequency
 Prevalence
 Incidence
 Relationship Between Prevalence and
Incidence
 Stratification of Disease Frequency by Person,
Place, and Time

3. Importance of Measures of Disease
Frequency
 Prior to any clinical trial:
How frequently does the disease occur?
 Provide big picture information about a
disease, framing public health questions and
guiding resource allocation.
 Describe the absolute risk of a disease.
 Can be categorized, or stratified to gain insight
into the pathogenesis (mechanism) of disease.

4. Prevalence
 The amount of a disease at one particular point
in time
 The proportion of people who have the disease
 Prevalence (%) =
×100%
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number of people in the population

5. Prevalence
 What is the prevalence of anxiety disorder
among UM medical student?
 What is the prevalence of cognitive disorders
among school children born at < 34 wks GA
in Manitoba?

6. Prevalence
Prevalence measures
help to describe the current burden of a
disease in a population in order to facilitate
planning and resource allocation

7. Incidence
 The number of new cases of disease
that develop over time

8. Incidence
 Two definitions
 Incidence proportion =
x 100%
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population without disease at baseline
 Incidence rate (Incidence density) =
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person-time at risk

9. Incidence
 What is the incidence of influenza
infection among UM medical students
(500 students) during a 3-month period
from January through March 2009?
 Time-at-Risk concept

10. Incidence
Diagram of individual risk time and disease status

11. Incidence
Incidence measures
help to provide clues as to the cause or
development of a disease

12. Relationship Between Prevalence
and Incidence

13. Stratification
 Refers to the process of separating analysis
by subgroups.
 Stratification can be by:
 Personal Characteristics: age, race/ethnicity, sex
 Geography
 Time periods

14. Reference
 Epidemiology and Biostatistics:
An Introduction to Clinical Research
Bryan Kestenbaum; editors, K. Adeney, N.
Weiss ; contributing author, Abigail B. Shoben
Springer Science+Business Media, LLC 2009