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EPIDEMIOLOGY
Lecture 2:
Measuring Disease Frequency
What did we learn in Lecture 1?
A quick Recap
• Definition of epidemiology
– “The study of the distribution and determinan...
Lecture Objectives
By the end of this lecture, the student will be
able to:
1. Describe the tools of measurements in
epide...
Definitions and terms
• Operational Definition:
– “A definition embodying criteria used to identify
and classify individua...
Tools of Measurement
in Epidemiology
RATIO
• “The value obtained by dividing one quantity
by another”
• Example: Sex ratio, Child-woman ratio
RATE
• “the frequency with which an event occurs in
a defined population, usually in a specified
period of time”.
• Exampl...
PROPORTION
• “The ratio of a part to the whole, expressed as
a “decimal fraction” (e.g., 0.2), as a “common
fraction” (1/5...
The FIVE elements (or components)
of a rate
Rate =
Number of events in a specified
population during a specified period
Av...
INCIDENCE AND PREVALENCE
INCIDENCE – WHAT IS IT?
• “the number of new health-related events in a
defined population within a specified period
of ti...
PREVALENCE – WHAT IS IT?
• “total number of individuals who have an attribute or
disease at a particular time (it may be a...
PREVALENCE
Definition of prevalence
• “total number of individuals who have an
attribute or disease at a particular time (it may be
a...
How is prevalence calculated?
Numerator
Denominator
Multiplier
• Point prevalence
– The proportion of individuals with a disease or an
attribute at a specified point in time.
Types of p...
• Period prevalence
• The proportion of individuals with a disease or an
attribute at a specified period of time.
• To cal...
• Lifetime prevalence
– The proportion of individuals who have had the
disease or condition for at least part of their liv...
Factors influencing prevalence
• Estimate the magnitude of health/disease
problems in the community and identify
potential high-risk population groups.
•...
• Prevalence studies do not usually provide
strong evidence of causality.
Disadvantages of prevalence
EXERCISES
• Calculate the prevalence of disease in the
given situations.
• SITUATION 1:
• In a population of 30000, 200 ad...
• SITUATION 2:
Jan 1,
2013
Dec 31,
2013
Population at risk =200
(on 1st July, 2013)
Epidemiology lecture 2 measuring disease frequency
Epidemiology lecture 2 measuring disease frequency
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Epidemiology lecture 2 measuring disease frequency

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Epidemiology
Measuring Disease Frequency-Prevalence

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Epidemiology lecture 2 measuring disease frequency

  1. 1. EPIDEMIOLOGY Lecture 2: Measuring Disease Frequency
  2. 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. 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. 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”.
  5. 5. Tools of Measurement in Epidemiology
  6. 6. RATIO • “The value obtained by dividing one quantity by another” • Example: Sex ratio, Child-woman ratio
  7. 7. RATE • “the frequency with which an event occurs in a defined population, usually in a specified period of time”. • Example: Incidence rate
  8. 8. 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.
  9. 9. 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
  10. 10. INCIDENCE AND PREVALENCE
  11. 11. 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”.
  12. 12. 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.
  13. 13. PREVALENCE
  14. 14. 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.
  15. 15. How is prevalence calculated? Numerator Denominator Multiplier
  16. 16. • Point prevalence – The proportion of individuals with a disease or an attribute at a specified point in time. Types of prevalence
  17. 17. • 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 …
  18. 18. • 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 …
  19. 19. Factors influencing prevalence
  20. 20. • 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
  21. 21. • Prevalence studies do not usually provide strong evidence of causality. Disadvantages of prevalence
  22. 22. 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?
  23. 23. • SITUATION 2: Jan 1, 2013 Dec 31, 2013 Population at risk =200 (on 1st July, 2013)

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