Basics of epidemiology

An updated presentation on the basics of epidemiology

1. National Institute of Health & Family Welfare Basics of Epidemiology Dr. Dipayan Banerjee DHA final year

2. Definition of Epidemiology The study of the distribution and determinants of disease or health related states and events in specified populations, and the application of the knowledge for disease prevention and control .

3. Clinical Practice Epidemiologist What is wrong with this patient ? What are disease in the population? How common are they? Who is most affected? History and Examination Population based surveys Why is this person ill? What are the risk factors for the condition of the population?

4. Investigations Epidemiological studies How shall I treat the patient What control strategies are needed to reduce this disease in population? Medication /Surgeries/ Nothing Intervention Studies Patients comes to you You have to got to Population or Source of data about the population Clinical Practice Epidemiologist

5. Components of Epidemiology Studies of Disease frequency Studies of the distribution. Studies of determinants.

6. Disease Frequency The basic measure of disease frequency is rate or ratio. These are essential for comparing disease frequency in different populations or subgroups of the same population in relation to suspected casual factors. Such comparisons may yield clues to disease etiology- which in turn is a vital step in the development of strategies for prevention or control of health problem.

7. Distribution of Disease It occurs in patterns in a community. The distribution patterns in various subgroups of the population should be studied by time, place and person. This study helps in formulation of etiological hypothesis .

8. Determinants of disease Identifying the underlying causes (or risk factors ) of disease. It helps in developing scientifically sound health programmes, interventions and policies It also helps in understanding the chronic disease causation .

9. Disease Causation – Triad An interaction of these three factors is required to start disease process in human beings.

10. Epidemiological Approach Asking Questions and making comparisons Related to health Events What is the problem?/ and Magnitude? Where did it happen? When did it happen? Who are affected? Why did it happen?

11. Cont.…… Questions Related to Health Action What can be done to reduce the problem? How can it be prevented? What action should be taken by the community? By the Health Services? By other sectors? What resources are required? How are the activities to be organized? What difficulties may arise in implementation?

12. Epidemiology and the Public's Health Descriptive Data Hypotheses Test Hypotheses Develop Interventions Test Interventions Intervene Surveillance Observational Studies Surveillance Intervention Studies

13. Measurements in Epidemiology Is very broad and unlimited and includes the following : Measurement of Mortality Measurement of Morbidity Measurement of disability Measurement of presence ,absence of disease or distribution of disease. Measure of medical needs ,health care needs ,health care facilities.

14. Tools of measurement Basic tool of measurement are: Rates: It measures the occurrence of some particular event in a population during a given period of time. Ratios: A relation in size between two random quantities. Proportions: It is a ratio indicting the relation in magnitude of a part of the whole.

15. Measures of disease frequency Prevalence All current cases (old & new) existing at a given point in time, or over a period of time in a given population. Incidence: Number of new cases, episodes or events occurring over a defined period of time.

16. Incidence Incidence may change genuinely (increase or decrease) with the following factors: Introduction of a new risk factor (e.g., contraceptive and increase in Thromboembolism, food additives and cancer); Changing habits. Changing virulence of causative organisms. Changing potency of treatment or intervention programs. Selective migration of susceptible persons to an endemic area, which increase the incidence of the disease.

17. Prevalence These rates are typically obtained from cross –sectional studies; occasionally, they are based on registers of specific disease. Prevalence depends on two main factors: - previous incidence ,and - Duration or chronicity of disease. When both incidence and during of a specific disease are relatively stable, Prevalence (P) = incidence (I) X duration (D)

18. Prevalence may change over time in response to: Changes in incidence, Changes in disease duration and chronicity (e.g., some disease may become shorter in duration or more acute because of high recovery rate or high case fatality rate), Intervention (preventive ) programs, Selective attrition (e.g., selective migration of cases ,or of susceptible or immune persons ), and Changing classification of what constitutes an “active ” case and whether an “ arrested” case is counted or not.

19. Measures of Central Tendency Mean : It is the sum of all the observations divided by the number of observations Median : It is that value of the variable which divides the distribution into two equal parts, one part containing values greater than the median and the other one containing values less than the median Mode : it the most occurring value

20. Data presentation Data can be Categorical or Qualitative- Male/Female , Numerical or Quantitative -Discrete data- observations in question can take only certain numerical values. e.g. Pulse rate – 80, 74, 64 -Continuous data- E.g. Weight of children in kg: 11.0, 10.8,

21. Ways of Data Presentation For Qualitative data Bar chart Pie chart For Continuous data Histogram Frequency polygon Cumulative Frequency curve

22. Results of Screening for diabetes on incidence rate 5 died 5 migrated 5 recovered 25 cases 40 cases 960 free 20 cases 940 free 1000 women Screening 31DEC77 1 January 1978 31 December 1978 Incident cases during 1978 Prevalent cases on 31 December Attrition

23. Describing the Occurrence of events Vital statistics -- births and deaths & marriages Disease prevalence Disease incidence Rates, ratios and proportions Indicators

24. Introduction to data triangulation Health programs generate data from multiple sources. But, different sections under the same program do review of the performance/service data separately. Thus fail to see the full picture and to correlate. Plan actions piecemeal Leading to incomplete achievements . DATA TRIANGULATION FACILITATES DECISIONS AND BETTER IMPLEMENTATION

25. Gather data from multiple sources Refine strategies for action Examine data quality& interpret Planning Triangulation Conducting Triangulation Communicating Results (for Action) A visual representation of the triangulation process

26. Steps in Data Triangulation Specify the questions Identify data sources, organize the data and identify data gaps Conduct data quality and validation checks Decide on data outlier and/or missing data Refine/revisit the questions chosen for data triangulation Analyze data from different sources for each question Do data triangulation & draw conclusions Summarize findings Plan strategies based on findings

27. DESCRIPTIVE EPIDEMIOLOGY Studies concerned with observing the distribution of disease or health related characteristics in human populations and identifying the characteristics with which the disease in question seems to be associated. These studies basically ask the following three questions  1.    When is the disease occurring? – Time distribution 2.    Where is it occurring? – Place distribution 3.    Who is getting the disease? – Person distribution

28. Time distribution Three kind of time trends or fluctuations have been identified 1.  Short term fluctuations 2. Periodic fluctuations 3. Long term or secular trends Short term fluctuations The best known Short term fluctuation in the occurrence of a disease is an epidemic

29. Types of Epidemics a . Common Source of Epidemics These epidemics originate from a single source of infection or disease producing agent. Two Common Source epidemics are described below: (I) Point source or Single Exposure Epidemic The agent responsible for the causation of disease is exposed to the agent population at risk at one point of time and only once. For instance, a marriage party may get an epidemic of food poisoning due to eating of contaminated food, the point source of the epidemic. The cases will occur within its incubation period. The time after which half the number of cases occur is know as the median incubation period. A single exposure epidemic may also be caused by a chemical (Bhopal Gas Tragedy) or a pollutant (Chernobyl nuclear holocaust).

30. (ii) Continuous or Multiple Exposure Epidemics The source of infection for the causation of an epidemic may be continuous. In such a case the epidemic shall not cease to exist till the source is removed. A well with contaminated water becomes a regular source of infection to the people using it and the epidemic may continue till the time the water is treated and made potable. b. Propagated Epidemics Infectious Epidemics The propagated epidemics are generally of infectious origin and the transmission of infection is from person to person. Vector transmission may also be there in diseases like malaria, cholera, gastroenteritis and conjunctivitis etc. The speed of spread depends on herd immunity, secondary attack rate and opportunities for contacts. These epidemics tend to show radial spread with the generation of a case.

31. Epidemic Curves Common source: Intermittent exposure (e.g., contaminated well water)

32. Seasonal Epidemics Infection diseases and some non-infectious diseases both tend to have an increased incidence during the summer months and similarly the respiratory infections during the winter months. Road traffic accidents are more frequent in rainy months and asthma in spring. These diseases take an epidemic dimension only during specific seasons.

33. Cyclical Epidemics Some epidemics tend to occur in cycles which may repeat themselves in weeks, months or years. Measles tends to present epidemics in a cycle of every 2-3 years. To break it, there is a need to cerate herd immunity . If the herd immunity is already there in the community, there is every likelihood that the diseases would not take epidemic form.

34. Uses of Epidemiology Establishing the magnitude and distribution of need Identifying high risk groups Health Situation Analysis/Health Status Assessment Planning provision of health care services Evaluate the impact of current services

35. Uses o f Epidemiology Identify factors , if they could be removed, would prevent the disease or modify its course Knowing which is the most effective way of delivering a curative or preventive measure Determine cost and expected effectiveness of a proposed strategy

36. How did we identify smoking as a cause of lung cancer? (How did we find a new cause of disease?)

37. Numbers of Lung Cancer Deaths, United States, By Year 1937 1938 1939 1940 1950 7,356 8,121 8,633 9,543 18,313

38. Cancer Death Rates By Site, U.S., 1930-1950 Males Females Rates are per 100,000 and adjusted to the 1970 U.S. Census Population

39. Tobacco Smoking and Lung Cancer: A Case-Control Study Smoker Non-smoker 597 8 Case 666 114 Control Source: Wynder and Graham, 1950

40. Never 1-14 15-24 25+ 0.07 0.57 1.39 2.27 Tobacco Smoking and Lung Cancer: British Physicians Study Level of Smoking: # Cigarettes/day Annual Death Rate (per 1,000) Source: Doll and Hill, 1964.