What is Epidemiology?

1. What is Epidemiology?

2. Lilienfeld A: in Foundations of Epidemiology THE STUDY OF THE DISTRIBUTION OF A DISEASE OR A PHYSIOLOGICAL CONDITION IN HUMAN POPULATIONS AND OF THE FACTORS THAT INFLUENCE THIS DISTRIBUTION (Note that epidemiology is applied to a broad variety of health related issues – not just infectious diseases.) Definitions of Epidemiology

3. Identify causes and risk factors for disease. Determine the extent of disease in the community. Study natural history and prognosis of disease. Evaluate preventive and therapeutic measures Provide foundation for public policy Purposes Of Epidemiology

4. What Is The Unique Skill Of Epidemiologists? MEASURING DISEASE FREQUENCY IN POPULATIONS

5. Classifying the disease Deciding what constitutes a case of disease in a study Finding a source for ascertaining the cases Defining the population at risk of disease Defining the period of time of riskof disease Obtaining permission to study people Making measurements of disease frequency Relating cases to population and time at risk Measuring Disease Frequency Has Several Components

6. Case Definition • case definition -defines a case in time, person and place. • Time criteria may include all cases of a disease identified from, for example, January 1, 2008 to March 1, 2008. • Person criteria may include age, gender, ethnicity, and clinical characteristics such as symptoms (e.g. cough and fever), clinical tests (e.g. pneumonia on chest X-ray). • Place criteria will usually include a geographical entity such as a town, state, or country but may be as small as an institution, a school class, or a restaurant meal session.

7. (Who? Where? When?) Examining the distribution of a disease in a population, and observing the basic features of its distribution in terms of person, place, and time. Typical study design: community health survey (approximate synonyms - cross-sectional study, descriptive study) (How? Why?) Testing a specific hypothesis about the relationship of a disease to a putative cause, by conducting an epidemiologic study that relates the exposure of interest to the disease of interest. Typical study designs: -cohort study -case-control study Two Broad Types of Epidemiology ANALYTIC EPIDEMIOLOGY DESCRIPTIVE EPIDEMIOLOGY

8. Cross Sectional Study • Basically this is a survey • Defines populations to be studied • Collects information from pop. members about their disease and exposure • “snapshot” of the population – point in time • Examines relationship between a variable and a disease – but does NOT establish cause and effect which requires data over time

9. The Basic Triad Of Descriptive Epidemiology THE THREE ESSENTIAL CHARACTERISTICS OF DISEASE WE LOOK FOR IN DESCRIPTIVE EPIDEMIOLOGY: • TIME • PLACE • PERSON Place Time Person

10. Changing or stable? Seasonal variation. Clustered (epidemic) or evenly distributed (endemic)? Point source or propagated. Time

11. Geographically restricted or widespread (pandemic)?   Relation to water or food supply.   Multiple clusters or one? Place

12. Age Socio-economic status Gender Ethnicity/Race Behavior Person

13. To undertake an analytic epidemiologic study you must first: Know where to look Know what to control for Be able to formulate hypotheses compatible with laboratory evidence Descriptive Epidemiology Is A Necessary Antecedent Of Analytic Epidemiology

14. The Basic Triad Of Analytic Epidemiology THE THREE PHENOMENAASSESSED IN ANALYTIC EPIDEMIOLOGY ARE: HOST AGENT ENVIRONMENT

15. Microbes=Classic view – others include: Nutrients – (ex rickets) Poisons Allergens Radiation Physical trauma(car crashes) Psychological experiences Agents

16. Genetics Immunologic state Age Personal behavior Host Factors

17. Influences interaction of host and agent Crowding Atmosphere/weather Modes of communication – phenomena in the environment that bring host and agent together, such as: Vector Vehicle Reservoir Environment

18. Public Health Strategies… • Depending on which public health strategy might be most effective, we might direct efforts at the specific agent (guinea worm), host ( immunization to prevent measles), or environment (sanitation improvements to prevent salmonella)

19. Analytical studies: CohortCase Control

20. Cohort Study • Follows population forward in time from suspected cause to effect • Begins with a group free of Disease. • Selects population according to exposure – Not whether they have the disease or health problem or not. Population is WELL DEFINED • See if the development of new cases differs between exposed and unexposed groups • Uses Relative Risk Calculation.

21. Cohort Study Example: A salmonella outbreak occurs at a company picnic…. • Each attendee is questioned about the food they ate (exposure) and whether they became ill afterward. • Relative risk for exposure can be CALCULATED (quantified) from the relationship between the exposure and outcome. • Relative Risk calculator at Medcalc/org

22. Case Control Study • Work backwards from Effect  Cause Case subjects thathave the disease are – compared to Control subjects who do not have the disease but are part of the same at-risk population. Population is not well defined as in Cohort studies You cannot calculate attack rates because you do not know the total number of people in the community who were and were not exposed to the source of the disease under study. Without attack rates, you cannot calculate relative risk Comparison of the two groups is quantified by an ODDS RATIO*: *See below in notes::.

23. Odds Ratio Calculator (Medcalc.org) • The odds ratio is used frequently in case/control studies • • In a case/control study, ill persons’ (“cases”) characteristics and exposures are compared with those of well persons (“controls”) selected from the same population in which the outbreak occurred.

24. Example: • in an outbreak suspected to stem from exposure to contaminated food at a restaurant, the ill persons’ food selections could be compared with those of some well people who also ate at the restaurant the same day. This could be done even if we didn’t know exactly how many people ate at the restaurant that day. • • A two-by-two table is constructed – • The OR is calculated by multiplying across the cells.

25. The formula for OR is: Odds Ratio = ad bc Where a = number of persons with disease and with exposure of interest b = number of persons without disease, but with exposure of interest c = number of persons with disease, but without exposure of interest d = number of persons without disease and without exposure of interest a+c = total number of persons with disease (“cases”) b+d = total number of persons without disease (“controls”)

26. Number of Cases of Disease X by Exposure History, Smallville, 2004 Disease X Yes No Total • Ate Tuna Casserole a =46 b= 25 71 • Didn’t Eat Tuna c= 18 d= 40 58 Casserole • The OR in this example is: • Odds Ratio = 46 x 40 = 1840 = 4.1 25 x 18 450 • So those who became ill were 4.1 times as likely to have eaten the tuna casserole. We should probably look a little more deeply into the tuna casserole!

27. EVERY HEALTH OUTCOME HAS SOME INTERESTING AND USEFUL EPIDEMIOLOGIC CHARACTERISTIC DEATH RATES BY SOCIAL CLASS FROM A CERTAIN CAUSE AMONG 1,316 PEOPLE WHAT CAUSE OF DEATH IS THIS?

28. The previous slide shows death rates by class of ticket on the Titanic, a large ocean liner that sank after colliding with an iceberg in 1912