basic measurements in ep.pptx presentation

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1. Basic measurements in epidemiology Dr. ChITRA TOMY , Asst. Professor, Dept. Of Community Medicine, AIMS, Kochi

2. Epidemiology... is the study of the distribution and determinants of health-related states or events in specified populations , and the application of this study to the control of health problems. JOHN.M.LAST Basic Measurements in Epidemiology 2

3. Why epidemiology? To effectively practice medicine and public health, we need evidence/knowledge on 3 fundamental types of professional knowing: DIA-gnosis------- ETIO-gnosis------- PRO-gnosis Basic Measurements in Epidemiology 3

4. Aims of Epidemiology To describe the magnitude & distribution of health and a disease To identify etiological factors in the pathogenesis of a disease To provide data essential to the planning implementation & evaluation of services Basic Measurements in Epidemiology 4

5. EPIDEMIOLOGICAL APPROACH Asking Questions Related to health events Related to health action Making Comparison To draw inferences Clues to aetiology Basic Measurements in Epidemiology 5

6. TOOLS OF MEASUREMENT: RATE RATIO PROPORTION This Photo by Unknown author is licensed under CC BY-NC . Basic Measurements in Epidemiology 6

7. RATE A rate measures the occurrence of some particular event (development of disease or the occurrence of death) in a defined population during a given time period.  It is a statement of the risk of developing a condition.  It indicates the change in some event that takes place in a population over a period of time.  example : death rate Basic Measurements in Epidemiology 7

8. RATE: Basic Measurements in Epidemiology 8

9. RATE: The various categories of rates are :  Crude rates: These are the actual observed rates such as the birth and death rates. Crude rates are also known as unstandardized rates. Specific rates: These are the actual observed rates due to specific causes (e.g., tuberculosis) ; or occurring in specific groups (e.g., age-sex groups) or during specific time periods (e.g., annual, monthly or weekly rates)  Standardized rates: These are obtained by direct or indirect method of standardization or adjustment, e.g., age and sex standardized rates Basic Measurements in Epidemiology 9

10. Rate: Basic Measurements in Epidemiology 10

11. RATE: Basic Measurements in Epidemiology 11

12. Incidence Rate The incidence rate of a disease is defined as the number of new cases of a disease that occur during a specified period of time in a population at risk for developing the disease . Measure of Morbidity Basic Measurements in Epidemiology 12

13. Prevalence: Measure of Morbidity All cases of disease in a community Basic Measurements in Epidemiology 13

14. Types of prevalence: Point prevalence: Prevalence at any given point of time. • 4% TB cases on 1st April Period prevalence: Prevalence at a given period of time . •Period will be 1year. Basic Measurements in Epidemiology 14

15. Prevalence vs. Incidence Prevalence:- how many people in a population currently have the disease (Photograph) Incidence :- how many people are diagnosed each year (Film) Basic Measurements in Epidemiology 15

16. the relationship... Basic Measurements in Epidemiology 16

17. RATIO: measure of disease frequency It expresses a relation in size between two random quantities. The numerator is not a component of the denominator.  The numerator and denominator may involve an interval of time or may be instantaneous in time.  Broadly, ratio is the result of dividing one quantity by another. It is expressed in the form of: Basic Measurements in Epidemiology 17

18. RATIO: Examples:  The ratio of white blood cells relative to red cells is 1:600 or 1/600, meaning that for each white cell, there are 600 red cells. sex-ratio, doctor- population ratio, child-woman ratio, Basic Measurements in Epidemiology 18

19. RATIO: Basic Measurements in Epidemiology 19

20. PROPORTION: A proportion is a ratio which indicates the relation in magnitude of a part of the whole.  The numerator is always included in the denominator.  No time factor  A proportion is usually expressed as a percentage. Basic Measurements in Epidemiology 20

21. Proportion: Basic Measurements in Epidemiology 21

22. CONCEPT OF NUMERATOR AND DENOMINATOR: Basic Measurements in Epidemiology 22

23. Numerator: Numerator refers to the number of times an event (e.g. , sickness, birth , death , episodes of sickness) has occurred in a population, during a specified time-period.  The numerator is a component of the denominator in calculating a rate, but not in a ratio. Basic Measurements in Epidemiology 23

24. Denominator It may be  (a) related to the population, or  (b) related to the total events Basic Measurements in Epidemiology 24

25. DENOMINATOR a. Related to the population: MID-YEAR POPULATION  POPULATION AT-RISK  PERSON-TIME PERSON DISTANCE  SUB-GROUPS OF THE POPULATION Basic Measurements in Epidemiology 25

26. DENOMINATOR  MID-YEAR POPULATION: Because the population size changes daily due to births, deaths and migration, the mid-year population is commonly chosen as a denominator. The mid-point refers to the population estimated as on the first of July of an year.  POPULATION AT-RISK: focuses on groups at risk of disease rather than on individuals.  For example, if we are determining the rate of accidents for a town, the population at risk is all the people in the town.  Similarly in calculating "general fertility rate", the denominator is restricted to women of child-bearing age (i.e., 15-49 years); older women and little girls are excluded because they are not "at risk" of becoming pregnant.  In short, '·population at risk" is restricted solely to those who are capable of having or acquiring the disease or condition in question Basic Measurements in Epidemiology 26

27. DENOMINATOR PERSON-TIME: In cohort studies, persons may enter the study at different times. Consequently, they are under observation for varying time periods.  In such cases. the denominator is  combination of persons and time. The most frequently used person- time is person- years.  this may be person- months, person-weeks or man-hours.  For example. if 10 persons remain in the study for 10 years, there are said to be 100 person-years of observation. The same figure would be derived if 100 persons were under observation for one year.   PERSON DISTANCE: A variant of person- time is person-distance, as for example passenger- miles.  SUB-GROUPS OF THE POPULATION: The denominator may be subgroups of a population, e.g., age, sex, occupation, social class, etc. Basic Measurements in Epidemiology 27