Measuring Population Health

1
Measuring Population Health
2
Learning Objectives

• To explain the difference between numerators and
  denominators, and their importance in population
  measurement
• To distinguish between different measures of
  health or disease frequency in populations

3
Rate

• In epidemiology, demography and vital statistics,
  a rate is an expression of the frequency with
  which an event occurs in a defined population in
  a specified period of time.
• The use of rates rather than raw numbers is
  essential for comparison of experience between
  populations at different times, different places
  or among different classes of persons.

4
Rate
(Numerator)
(Denominator)

• The components of a rate
• Numerator
• Denominator
• Specified time in which the events occur
• a multiplier (a power of 10) that converts the
  rate from an awkward fraction or decimal to a
  whole number

5
Incidence Rate

• Number of new cases of disease over a period of
  time
• __________________________________
• Population at Risk

6
Incidence Rates

• Need
• A period of observation
• Numerator (number of new cases)
• Times of disease onset (diagnosis)
• Denominator (Population at risk)

7
Measures of Morbidity

• Incidence measures the rate at which people
  without the disease develop the disease during a
  specified period of time
• Used to study disease aetiology (risk)

8
Infection in a Population
Low incidence constantly present
High incidence at set time, above endemic levels
Global epidemics
9
Prevalence Rate

• Number of existing cases of disease at a point in
  time
• ___________________________________
• Total Population

10
Prevalence

• Prevalence measures the number of people in a
  population who have the disease at a given point
  in time
• Depends on Incidence and duration
• P I X D
• Used to measure disease burden

11
Prevalence Rates

• Need
• Definite point in time
• Numerator (number of existing cases)
• Time of disease onset
• Denominator (size of population)

12
Factors that Influence Prevalence

• Change in Incidence ( )
• Longer duration of the disease
• Prolongation of life of patients without cure
• Change in case fatality rate
• Selective migration
• In-migration of cases, out-migration of healthy
  people
• In-migration of susceptible people
• Improved diagnostic facilities
• Primary and Secondary Prevention strategies

13
Relationship Between Incidence and Prevalence

• Prevalence depends on incidence
• Higher incidence leads to higher prevalence if
  duration of cases does not change.
• Limitation of the bathtub analogy flow rate
  needs to be expressed relative to the size of the
  source
• Prevalence Incidence x Duration of disease

14
The Bathtub Analogy
15
Trends
Secular trend refers to a change in the
prevalence of infection over years. This relates
to better living conditions, better hygiene, and
vaccination. An example of a secular trend is the
decrease in tuberculosis (TB) in UK. Seasonal
trend refers to changes in the prevalence of
infection occurring over the year, e.g., flu
outbreaks - the reason the seasonality is unclear
but changes of temperature, crowding and humidity
may play a role.
16
References

• Beaglehole, R., Bonita, R., and Kjellström, T.
  (1993) Basic Epidemiology. 2nd ed. World Health
  Organisation 2006.
• Farmer, R., Miller, D., Lawrenson, R.
  (2000)Lecture Notes on Epidemiology and Public
  Health Medicine, 4th Edition, Blackwell Science.
• HOUSTON, J.C., JOINER, C.L., and TROUNCE, J.R.
  (1979) A short textbook of medicine. 6thed.
  Sevenoaks Hodder and Stoughton.
• Rothman, KJ., Greenland, S. (1998) Modern
  Epidemiology, 2nd Edition, Lippincott Williams
  and Wilkins.