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Epidemiology lecture 2 measuring disease frequency
Measuring Disease Frequency
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
By the end of this lecture, the student will be
1. Describe the tools of measurements in
2. Enumerate the FIVE elements in a rate.
3. Define and differentiate between incidence and
4. Enumerate types of prevalence.
5. Calculate the prevalence of a disease/health-
related behavior in a population.
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
• “The value obtained by dividing one quantity
• Example: Sex ratio, Child-woman ratio
• “the frequency with which an event occurs in
a defined population, usually in a specified
period of time”.
• Example: Incidence rate
• “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
The FIVE elements (or components)
of a rate
Number of events in a specified
population during a specified period
Average population during
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”.
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
• It is a proportion, not a rate.
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
• It refers to NEW+OLD cases of the disease.
How is prevalence calculated?
• Point prevalence
– The proportion of individuals with a disease or an
attribute at a specified point in time.
Types of prevalence
• 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
Types of prevalence – contd …
• 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 …
• 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
• Prevalence is a useful measure of the occurrence
of conditions for which the onset of disease may
be gradual, such as hypertension, diabetes or
Uses of prevalence
• Prevalence studies do not usually provide
strong evidence of causality.
Disadvantages of prevalence
• Calculate the prevalence of disease in the
• 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?
• SITUATION 2:
Population at risk =200
(on 1st July, 2013)