Evidence Based Chronic Disease Prevention Module Three : Quantifying the Issue

1. Evidence Based ChronicDisease PreventionModule Three:Quantifying the Issue Presented by: Karen Peters, DrPH

2. Objectives • Understand the basic concepts of descriptive epidemiology • Understand the major designs and contributions of analytic epidemiology • Be more knowledgeable of several major sources of public health surveillance data. Evidence Based Chronic Disease Prevention

3. Epidemiology • The basic science and fundamental practice of public health and preventive medicine • Epidemiology comes from epidemic - in Greek ‘upon the people’ • Hippocrates, Airs, Waters, Places…Consider the context in which people live, the airs ‘peculiar to each particular region’, the ‘properties of the waters’, and ‘the mode of life of the inhabitants, whether they are heavy drinkers, taking lunch, and inactive, or athletic, industrious, eating wisely and drinking little.’ Evidence Based Chronic Disease Prevention

4. Uses of Epidemiology • Historical study: Is community health getting better/worse? • Community assessment: What actual and potential health problems are there? • Working of health services (Efficacy, Effectiveness, Efficiency) • Individual risk and chances (Actuarial risks, Health hazard appraisal) • Completing the clinical picture: Different presentations of a disease • Identification of syndromes • Search for causes: case control and cohort studies • Evaluation of presenting symptoms and signs • Clinical decision analysis Evidence Based Chronic Disease Prevention

5. Epidemiologic Reasoning • Disease is not randomly distributed in a population. • Measurable factors influence the pattern of disease and the underlying causes of disease • Disease causation is multi-factorial. • Multiple factors interact to create an environment in which disease occurs. Evidence Based Chronic Disease Prevention

6. Epidemiologic Reasoning • Epidemiology is the study of the distribution and determinants of health-related states and events in specified populations and the application of this study to the control of health problems. Evidence Based Chronic Disease Prevention

7. Epidemiologic Reasoning: • Distribution • Patterns of disease within or across defined populations • Relates to time, place and person • Descriptive epidemiology Evidence Based Chronic Disease Prevention

8. Epidemiologic Reasoning: • Determinants • Factors that influence the prevention, occurrence, control, and outcome of disease • 2 types of determinants: host and environmental • Host: determine susceptibility of individual (e.g. age, sex, genetics, race, physiologic state, nutrition state) • Environmental: determine hosts exposure to specific agents (e.g. conditions of living, family size, composition, crowding, geography, climate) • Analytic epidemiology Evidence Based Chronic Disease Prevention

9. General Considerations • Major public health planning data use rates/ratios that must be evaluated when making health planning and policy decisions • Rates/ratios provide basis for most public health assessment and are integral to analyzing process and outcome measures Evidence Based Chronic Disease Prevention

10. Measures of Disease Frequency • Two Types of Frequency Measures • Attribute Data • Counts • Variable Data • Proportions • Ratios • Rates Evidence Based Chronic Disease Prevention

11. Measures of Disease Frequency • Count = number of events » 15.7 million diabetic cases (nationally – ADA) • Attribute data Evidence Based Chronic Disease Prevention

12. Measures of Disease Frequency: Proportion • A proportion is the expression of one part to the whole • Proportion (part to whole): a/a+b • Numerator: People with the disease, condition or event • Denominator: All people (with and without the disease, condition or event) • Numerator is always included in the denominator - females/females + males Evidence Based Chronic Disease Prevention

13. Measures of Disease Frequency: Proportion • Proportion = number of events divided by the population from which the events occurred » 15,700,000 diabetic cases 266,100,000 population = 0.059 diabetic cases* (5.9%) • variable data * Percentage is a proportion multiplied by 100 percent Evidence Based Chronic Disease Prevention

14. Measures of Disease Frequency: Ratio • A ratio is always defined as a part divided by another part • Ratio (part to part): a/b • Numerator: People with the disease, condition or event • Denominator: People without the disease, condition or event • Numerator is not included in the denominator - females/males Evidence Based Chronic Disease Prevention

15. Measures of Disease Frequency: Ratio • Ratio = number of events divided by another number » 15.7 million individuals with diabetes 250.4 million individual without diabetes • variable data Evidence Based Chronic Disease Prevention

16. Measures of Disease Frequency: Rate • A rate measures the occurrence of an event in a population over time • Rate (part to whole over time): (a/a + b) x k • Numerator: People with the disease, condition or event in a given period • Denominator: All people (with and without the disease, condition or event) • Numerator: Number of individuals something happened to in the population-at- risk- deaths/population x 10,000 = deaths per 10,000 Evidence Based Chronic Disease Prevention

17. Measures of Disease Frequency: Rate • Remember: • Cases in the numerator must come from the population-at-risk • The denominator must be the population-at-risk for the event (should have been able to experience the event) • Numerator and denominator should cover the same time period • All rates are multiplied by an appropriate constant (i.e., 100; 1,000; 100,000) Evidence Based Chronic Disease Prevention

18. Measures of Disease Frequency: Rate • Rate = number of events divided by the population from which the events occurred, during a defined period » 15,700,000 diabetic cases in one year 266,100,000 population = 0.059 diabetic cases per year • variable data Evidence Based Chronic Disease Prevention

19. Measures of Disease Frequency:Rate • Rates are usually expressed as a whole number for a given population during a specified period. 0.059 diabetic cases per year x 100,000 = 5,900 diabetic cases / 100,000 individuals / year Evidence Based Chronic Disease Prevention

20. Measures of Disease Frequency: Rate • Questions to ask about a rate: • What kind of rate is it? (crude, stratified, adjusted) • What is it a rate of? (mortality, morbidity, fertility) • To what population or group does the rate refer? (US, IL, county, children, adults, older adults) • How was the information obtained? (vital records, census, hospital discharge, BRFSS, face to face, telephone, paper/pencil) Evidence Based Chronic Disease Prevention

21. Guidelines for Using Rates • Ecological fallacy - generalizing data collected in a particular area, to all the individuals living in that area • derive indicators from a denominator that includes the entire population group • apply indicators with subgroup denominators only to the persons in that subgroup • Variations in Base - rates should be accompanied by an indication of its base to be meaningful • Ask, “rate per what?” Evidence Based Chronic Disease Prevention

22. Guidelines for Using Rates • False Association - Forgetting that rates apply to aggregates not individuals • don’t assume each individual possesses the characteristics shown to be associated at the neighborhood level • Small denominators and variance of rates - As population base decreases, statistical variation becomes more prominent as an explanation for observed differences • Calculate confidence limits Evidence Based Chronic Disease Prevention

23. Proportions, Ratios and Rates Again! • Not 3 distinctly different kinds of frequency measures • All are ratios • Proportions are a particular type of ratio • Some rates are a particular type of proportion • Numbers are converted into proportions, ratios and rates to generate comparable indices • EX: #/deaths for 2 groups at 2 different times, # differs due to size of population at risk - converting to rate (deaths/10,000) effect size is removed and rates can be compared Evidence Based Chronic Disease Prevention

24. Measures of Disease Frequency: Rates • Question: Do rates distinguish between existing and new cases of diabetes? Evidence Based Chronic Disease Prevention

25. Measures of Disease Frequency: Prevalence vs. Incidence • Prevalence is the number of existing cases of diseases in a population during a defined period. [15.7 million people (5.9%) have diabetes] • Incidence is the number of new cases of disease that develop in a population at risk during a defined period. [798,000 new cases diagnosed per year] Evidence Based Chronic Disease Prevention

26. Measures of Disease Frequency: Prevalence vs.. Incidence • Example: National prevalence rate of diabetes =15.7 million diabetic cases/year 266.1 million individuals = 0.059 diabetic cases per year (5.9%) • Example: National incidence rate of diabetes = 798, 000 new diabetic cases in 1 year 266, 100, 000 individuals = 0.0030 diabetic cases per year x 100,000 = 300 new diabetes cases/100,000/year Evidence Based Chronic Disease Prevention

27. Measures of Disease Frequency: Prevalence • Includes newly diagnosed (new) and prior surviving (or existing) cases of disease in a population during a defined period. • Sometimes called point prevalence, period prevalence, or “prevalence rate.” Evidence Based Chronic Disease Prevention

28. Measures of Disease Frequency: Point Prevalence • Point Prevalence: attempts to measure disease a one point in time • When term ‘prevalence’ is used usually refers to ‘point prevalence’ • # existing cases of a disease at a point in time Estimated total population at same point in time Evidence Based Chronic Disease Prevention

29. Measures of Disease Frequency: Period Prevalence • Period Prevalence: compound measure, constructed from prevalence at a point in time, plus new cases (incidence) and recurrences during the time period (over one year) • # of existing cases of a disease during interval Estimated population at mid- interval (Average population) Evidence Based Chronic Disease Prevention

30. Measures of Disease Frequency: Prevalence • Prevalence is a function of both incidence and duration of the disease. • P = I x D (when I and D are fairly stable) • Rare diseases, (e.g., diabetes), can have relatively high prevalence, if disease duration is long. • Common diseases, (e.g., head lice), can have moderate or low prevalence, if duration is short. Evidence Based Chronic Disease Prevention

31. Characteristics of Prevalence • Cause and effect measured simultaneously • Impossible to infer causation • Useful for planning (e.g. beds, clinics, workforce needs) • High prevalence  high risk • could reflect increased survival(improved care, behavior change - long duration) • Low prevalence  could reflect rapid fatal or cure process - short duration) • Easy to obtain need only 1 measurement Evidence Based Chronic Disease Prevention

32. Measures of Disease Frequency: Prevalence • Question: How is the prevalence affected? • A substantial improvement in treatment for those with diabetes (example: pancreatic  islet cell transplant) • A large influx of immigrants with higher rates of diabetes. Evidence Based Chronic Disease Prevention

33. Measures of Disease Frequency: Incidence • 2 major measures of incidence • Cumulative incidence • Incidence density Evidence Based Chronic Disease Prevention

34. Measures of Disease Frequency: Cumulative Incidence • Measures the proportion of individuals in the population who develop the disease within a defined period. • This is a measure of risk, or the probability that an individual in the population will develop the disease over a specified period, assuming that the individual does not die from any other disease during the same period. • # new cases dx reported during a time interval Population at risk at start of time interval Evidence Based Chronic Disease Prevention

35. Cumulative Incidence • Cumulative = number of new cases of disease incidence in the total population at risk • = 798, 000 new diabetic cases during one year 266, 100, 000 individuals = 0.0030 = 300 diabetes cases/100,000 individuals / year • Question: Can we follow all individuals in the population for one year to determine whether or not they develop diabetes? Evidence Based Chronic Disease Prevention

36. Incidence Density • Measures the rate at which new cases of disease occur in the population at risk during a defined period • The population at risk is dynamic and each person in the population contributes the amount of time that they remained under observation and free from disease (person-time) Evidence Based Chronic Disease Prevention

37. Incidence Density • For incidence density, the denominator is measured in person-time units rather than persons, which accounts for… • Persons who enter the population after the study period begins, • Persons who are ‘lost’ during the study period, and • Persons who develop the disease during the study period and are no longer at risk of developing the disease Evidence Based Chronic Disease Prevention

38. Incidence Density • Total person-time for the denominator is computed by either… • Summing the amount of person-time contributed by each person in the population during the study period, or • Multiplying the average size of the population at the mid-point of the study period times the number of years representing the total study period Evidence Based Chronic Disease Prevention

39. Incidence Density • Incidence Density • = number of new cases of disease in one year Total person-years • = 798, 000 new diabetic cases during one year 266, 100, 000 person-years = 0.0030 = 300 diabetes cases/100,000 person-years Evidence Based Chronic Disease Prevention

40. Characteristics of Incidence • Direct indicator of risk • high incidence = high risk of disease • Low incidence usually means low risk of disease • Incidence rates often underestimated • Incomplete reporting • Subject to seasonal variation Evidence Based Chronic Disease Prevention

41. Exercise: Calculation of person-time for incidence density • Go To Handout #1: Module 4 • Figure out # of years each person represents • Figure out # of cases during study period • Incidence density = # cases/person-time x 100 Evidence Based Chronic Disease Prevention

42. Solution for Incidence Density Calculation • Person A: 5.0 • Person B: 3.0 • Person C: 2.0 • Person D: 4.0 • Person E: 1.5 • Total: 15.5 person years • Incidence = 2 cases/15.5 person-years x 100 • = 12.9/100 person-years of observation Evidence Based Chronic Disease Prevention

43. Incidence # new cases of dx during a specific period of time measure # of new cases of dx in pop during specific time period dynamic concept/ measures risk Prevalence # existing cases of DX during particular point in time measure # of individuals with dx in pop at a given point in time static concept/ measures burden Incidence and Prevalence: Compared and Contrasted Evidence Based Chronic Disease Prevention

44. Measures of Disease Frequency • Question: Is it prevalence (what type) or incidence? • Do you currently have asthma? • Have you had asthma during the last 5 years? • Have you ever had asthma? Evidence Based Chronic Disease Prevention

45. Measures of Disease Frequency:Primary/Secondary Attack Rates • Primary Attack Rate: • # new cases of dx reported during an epidemic Population at start of epidemic period • Secondary Attack Rate: • # new cases dx among contacts of known cases Size of contact population at risk Evidence Based Chronic Disease Prevention

46. Rate Type Crude Category-specific Standardized Rate Meaning Un-adjusted Stratified Adjusted Interpretation of Rates Evidence Based Chronic Disease Prevention

47. Measures of Disease Frequency: Crude (or unadjusted) rates • Estimate the actual disease frequency for a population • Can be used to provide data for allocation of health resources and public health planning • Can be misleading if compared over time or across populations. • For example, the number of persons nationally with diagnosed diabetes increased from 1.6 million in 1958 to 8 million in 1995 a fivefold increase. Evidence Based Chronic Disease Prevention

48. Measures of Disease Frequency: Crude (or unadjusted) rates • For example, the number of persons nationally with diagnosed diabetes increased from 1.6 million in 1958 to 8 million in 1995 a fivefold increase. Evidence Based Chronic Disease Prevention

49. Measures of Disease Frequency: Category-specific (or stratified) rates • Category specific (stratified rates) • are “crude rates” for subgroups of the total population. For diabetes: • 6.3 million (18.4%) of all people age 65 or older • 11.3 million (7.8%) of all non-Hispanic whites • 2.3 million (10.8%) of all non-Hispanic blacks • 1.2 million (10.6%) of all non-Hispanic Mexican Americans • 8.1 million (8.2%) of all women have diabetes Evidence Based Chronic Disease Prevention

50. Measures of Disease Frequency: Category-specific (or stratified) rates • Provide more detailed information than crude rates about patterns of disease frequency in different populations • Can be used for valid comparison of populations • Can be cumbersome if there is a large number of categories to compare. Evidence Based Chronic Disease Prevention