Incidence And Prevalence

1. Incidence and Prevalence
Measures of Disease Frequency
Ayman Abou Mehrem, MD, CABP
Neonatology Academic Day
March 24, 2010

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%
number of people with disease_____________________________
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%
Incidence rate (Incidence density) =
number of new cases of disease_______________________________
population without disease at baseline
number of new cases of disease_____________________________
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