Epidemiology Measuring Disease Frequency-Prevalence

1. EPIDEMIOLOGY
Lecture 2:
Measuring Disease Frequency

2. What did we learn in Lecture 1?
A quick Recap
• Definition of epidemiology
– “The study of the distribution and determinants of health-related states or
events in specified populations, and the application of this study to the
prevention and control of health problems”
• The Epidemiological Approach
– Asking Questions
– Making comparisons
• The aims of epidemiology
– Describe disease – Distribution, frequency
– Identify etiological factors
– Provide data for planning and implementation and for priority setting

3. Lecture Objectives
By the end of this lecture, the student will be
able to:
1. Describe the tools of measurements in
epidemiology.
2. Enumerate the FIVE elements in a rate.
3. Define and differentiate between incidence and
prevalence.
4. Enumerate types of prevalence.
5. Calculate the prevalence of a disease/health-
related behavior in a population.

4. Definitions and terms
• Operational Definition:
– “A definition embodying criteria used to identify
and classify individual members of a set or
concept to facilitate classification and counting.”
• Population at risk:
– “People who are potentially susceptible to the
disease under study”.
– “The group of people, healthy or sick, who would
be counted as cases if they had the disease being
studied”.

6. Tools of Measurement
in Epidemiology

7. RATIO
• “The value obtained by dividing one quantity
by another”
• Example: Sex ratio, Child-woman ratio

8. RATE
• “the frequency with which an event occurs in
a defined population, usually in a specified
period of time”.
• Example: Incidence rate

9. PROPORTION
• “The ratio of a part to the whole, expressed as
a “decimal fraction” (e.g., 0.2), as a “common
fraction” (1/5), or as a percentage (20%)”. The
proportion is a dimensionless quantity.
• Example: Proportion of diabetics in a
population.

10. The FIVE elements (or components)
of a rate
Rate =
Number of events in a specified
population during a specified period
Average population during
the period
X 10
n
Numerator
Denominator
Multiplier
Time
Population

11. INCIDENCE AND PREVALENCE

12. INCIDENCE – WHAT IS IT?
• “the number of new health-related events in a
defined population within a specified period
of time. It may be measured as a frequency
count, a rate, or a proportion”.

13. PREVALENCE – WHAT IS IT?
• “total number of individuals who have an attribute or
disease at a particular time (it may be a particular
period) divided by the population at risk of having
the attribute or disease at that time or midway
through the period”.
• A measure of occurrence or disease frequency, often
used to refer to the proportion of individuals in a
population who have a disease or condition.
• When used without qualification, the term usually
refers to the situation at a specified point in time
(point prevalence).
• It is a proportion, not a rate.

15. PREVALENCE

16. Definition of prevalence
• “total number of individuals who have an
attribute or disease at a particular time (it may be
a particular period) divided by the population at
risk of having the attribute or disease at that time
or midway through the period”.
• A measure of occurrence or disease frequency,
often used to refer to the proportion of
individuals in a population who have a disease or
condition.
• It refers to NEW+OLD cases of the disease.

17. How is prevalence calculated?
Numerator
Denominator
Multiplier

18. • Point prevalence
– The proportion of individuals with a disease or an
attribute at a specified point in time.
Types of prevalence

19. • Period prevalence
• The proportion of individuals with a disease or an
attribute at a specified period of time.
• To calculate a period prevalence, the denominator
used most commonly is the ‘population at risk
midway through the period’ (e.g., mid-year
population)
Types of prevalence – contd …

20. • Lifetime prevalence
– The proportion of individuals who have had the
disease or condition for at least part of their lives
at any time during their lifecourse.
Types of prevalence – contd …

21. Factors influencing prevalence

22. • Estimate the magnitude of health/disease
problems in the community and identify
potential high-risk population groups.
• Assessing the need for preventive action,
healthcare and the planning of health
services.
• Prevalence is a useful measure of the occurrence
of conditions for which the onset of disease may
be gradual, such as hypertension, diabetes or
rheumatoid arthritis.
Uses of prevalence

23. • Prevalence studies do not usually provide
strong evidence of causality.
Disadvantages of prevalence

24. EXERCISES
• Calculate the prevalence of disease in the
given situations.
• SITUATION 1:
• In a population of 30000, 200 adolescents girls
are anemic. Calculate the prevalence of
anemia among adolescent girls if adolescent
girls comprise 10% of the population?

25. • SITUATION 2:
Jan 1,
2013
Dec 31,
2013
Population at risk =200
(on 1st July, 2013)