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Thomas Songer, PhD

Introduction to Research Methods In the Internet Era. Introduction to Epidemiology. Epidemiology as a Population Science Basic Epidemiology Measures. Thomas Songer, PhD. Key Lecture Concepts. Understanding epidemiology as a science focused on populations

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Thomas Songer, PhD

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  1. Introduction to Research Methods In the Internet Era Introduction to Epidemiology Epidemiology as a Population Science Basic Epidemiology Measures Thomas Songer, PhD

  2. Key Lecture Concepts • Understanding epidemiology as a science focused on populations • Samples of the population are taken to assess health issues • Health outcomes data can be expressed through multiple measures • These measures can be expressed as differing metrics 2

  3. What is Epidemiology? 3

  4. Epidemiology • Epidemiology is the study of the determinants, distribution, and frequency of disease in human populations • Who gets disease and why • Epidemiologists study sick and well people to determine the crucial difference between those who get disease and those who are spared 4

  5. Purpose of Epidemiology • To provide a basis for developing disease control and prevention measures for groups at risk. This translates into developing public health measures to prevent or control disease. 5

  6. Population Focus The focus of epidemiology is on the occurrence of health and disease in the population.  What happens to many The population approach contrasts with clinical medicine’s primary concern with health and disease in the individual.  What happens to one 6

  7. Health as a manifestation of individuals in social groups • Humans are social animals. • Many diseases are caused only by the interaction of individuals within and between populations. • Disease and health outcome patterns are generated in and by populations and need to be described, explained and predicted in a population setting. Bhopal 2002 7

  8. What is a Population? The common definition of a population is “All the inhabitants of a given country or area considered together;…”  A “population” can also be groups of individuals that share a common thread Clinical populations Subgroups of the population by age, race, etc… 8

  9. Epidemiology as a population science • Diseases are expressed biologically in individuals, however, no epidemiological study can be done on one person • Epidemiology studies humans in the aggregate (i.e. groups) • Conclusions are directly applicable to the groups studied • Conclusions are only indirectly applicable to individuals Bhopal 2002 9

  10. Epidemiology is… • The study of disease and its treatment, control, and prevention in a population of individuals. • Whole populations may be examined, but… • More frequently, samples of the population may be examined. Samples that are studied must be representative of the population for the results to be generalized to the total population. Torrence 1997 10

  11. Descriptive & Inferential Statistics Descriptive Statistics deal with the enumeration, organization and graphical representation of data from a sample Inferential Statistics deal with reaching conclusions from incomplete information, that is, generalizing from the specific sample Inferential statistics use available information in a sample to draw inferences about the population from which the sample was selected Rahbar

  12. Background • Different types of activities and practices are undertaken in epidemiology to develop disease control and prevention measures for groups at risk. 12

  13. Broad Characterizations of Epidemiology Practices • Descriptive Epidemiology • Examining, identifying, and reporting on the frequency and distribution of disease in a population. Learning the basic features of its distribution. • Analytic Epidemiology • Identifying factors underlying disease or health events. Testing a hypothesis by studying how exposures relate to outcomes 13

  14. Broad Characterizations of Epidemiology Practices • Developing interventions to reduce disease or improve health in the community • Using information from analytical studies, develop strategies centered around an important exposure factor. Test these strategies with clinical trials. • Program Evaluation • Examining the effectiveness of programs for disease control in the community 14

  15. There is a logical sequence to the practice of epidemiology in disease prevention Descriptive Analytical Interventions Programs Disease prevention 15

  16. Basic Question in Research Are exposure and disease/outcome linked? Is there an association between them? E D Exposure Disease / Health Outcome 16

  17. Health outcomes in research studies may be expressed through multiple types of measures 17

  18. Basic Measurements of Disease or Health Outcome Frequency in Epidemiology Measurement of Mortality (death) Measurement of Morbidity (incidence, prevalence) 18

  19. The development of new cases of a disease that occur during a specified period of time in previously disease-free or condition-free (“at risk”) individuals. Incidence 19

  20. There are two fundamental approaches to considering the incidence of disease or a health condition - Cumulative Incidence - Incidence Rate (Incidence Density) Approaches 20

  21. Prevalence: is another major measure of disease in the population It quantifies the “burden” of disease Number of existing cases of disease in population in time period Prevalence Rate = Persons in population in same time period 21

  22. Epidemiology is frequently used in clinical populations…. • To identify the response of health problems to health care solutions • Assess the impact of health care on health problems (i.e. does treatment work?) • Assist in the development of health services and programs 22

  23. Health Outcomes • May be intermediate in the clinical course of a disease or treatment • Short-term events • May be the end-result of the clinical course of a disease or treatment • Longer-term events 23

  24. Health Outcomes Related to Prognosis or the Evaluation of Health Care Interventions • Death • Recovery • Ongoing Disease • Stable disease with treatment • Progressive disease • disability • HRQOL; health related quality of life • Re-infection, Recurrence 24

  25. Mortality: is one of the major measures of disease in the population • information available from death certificates (required by law) • Crude Death rate: Number of deaths in time period = Number at risk of dying in period 25

  26. Infant Mortality Rate: assesses the risk of dying during the first year of life Number of deaths under age 1 year in time period = Number of live births in time period 26

  27. Case-fatality: the frequency in which cases of disease die • Case-fatality rate: the proportion of persons who die from a disease Number of deaths among persons with disease = Number of persons with the disease 27

  28. Years of Potential Life Lost (YPLL):Measure of the loss of future productive years resulting from a specific cause of death. Indicates the potential burden related to when death occurs. YPLL are highest when: • The cause of mortality is common or relatively common, AND • Deaths occur at an early age. 28

  29. Other Health Outcomes Disability rates Nutrition-related indicators Health Care Utilization rates Indicators of social and mental health 29 Salama

  30. Outcome measures in research studies may be expressed in differing units, though most often as rates 30

  31. Tools of Measurement • Counts • Proportions • Ratios • Rates 31

  32. Case Counts • Measuring disease or health or health care frequency startswith counting cases • Simplest and most frequently gathered measure in epidemiology 32

  33. Counts • Refers to the number of cases of a disease or other health phenomenon being studied i.e. Number of cases of influenza in Astana in January 2012 • Can be useful for allocation of health resources • Limited usefulness for epidemiologic purposes without knowing size of the source population 33

  34. Counts – Limited Interpretation New Cases Locationof DiseaseYearPopulation City A 20 2008 100 City B 100 2008 1000 Annual Rate of Occurrence City A: 20 / 100 = 1 / 5 City B: 100 / 1000 = 1 / 10 34

  35. Proportions • Persons included in the numerator are always included in the denominator: A Proportion: -------- A + B • Indicates the magnitude of a part, related to the total. In epidemiology, tells us the fraction of the population that is affected. 35

  36. Proportions - Example P = A / (A + B) = (1,400 / 11,050) = 0.127 36

  37. Ratios • Like a proportion, is a fraction, BUTwithout a specified relationship between the numerator and denominator • Example: Occurrence of Major Depression Female cases = 240 240 ------------------------ = ---- 2:1 female to male Male cases = 120 120 37

  38. Rates • A ratio in which TIMEforms part of the denominator • Epidemiologic rates contain the following elements: • health issue frequency (in the numerator) • unit size of population • time period during which an event occurs 38

  39. Rate: a measure of the occurrence of a health event in a population group at a specified time period Number of events in time period numerator : denominator Number at risk for the event in period 39

  40. Rates are the basic tool of epidemiologic practice • Why are rates important? • because they provide more complete information to describe or assess the impact of a health issue in a community or population 40

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