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. Low incidence constantly present High incidence at set time, above endemic levels Global epidemics Infection in a Population

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. 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. Trends

16. References:  Beaglehole, R., Bonita, R., and Kjellström, T. (1993) Basic Epidemiology. 2 nd ed. World Health Organisation 2006.  Farmer, R., Miller, D., Lawrenson, R. (2000)Lecture Notes on Epidemiology and Public Health Medicine, 4 th 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, 2 nd Edition, Lippincott Williams and Wilkins.