1. Measuring the Occurrence of Disease 1 Sue Lindsay, Ph.D., MSW, MPH Division of Epidemiology and Biostatistics Institute for Public Health San Diego State University

2. Pandemic Preparedness www.readysandiego.org http://www.ready.gov

3. Types of Measurement Commonly Used in Epidemiology Proportions 25 males/50 people = 0.50 or 50% Rates number of cases/population, cohort, or sample a specified time period all in the denominator have the potential to be in the numerator Ratios An expression of the relationship between numerator and denominator where they are two separate and distinct quantities: 25 females/29 males

4. Common Sources of Data For Stages in the Natural History of Disease Disease Onset SymptomsSeek Care Diagnosis/ Treatment Healthy Interviews Medical Records Sources Of Data } Outcome Surveys Birth Records Death Records

5. Measures of Morbidity Point Prevalence Period Prevalence Incidence Cumulative incidence Incidence density Lifetime incidence

6. Types of Populations Dynamic Population Fixed Population

7. Point Prevalence Prevalence Per 1,000 = No. of Existing Cases of Disease In a Population No. of Persons in the Population At a Specific Point in Time Expressed as either a rate or percentage

8. Period Prevalence Prevalence Per 1,000 = No. of Existing Cases of Disease In a Population No. of Persons in the Population During a Specified Period of Time Expressed as a rate: 50/10,000 persons in City A have diabetes in 2007

9. Incidence Per 1,000 = No. of New Cases of Disease In a Population No. of Persons at Risk of Developing Disease During a Specified Period of Time Expressed as a rate: 10 persons/10,000 developed new diabetes in 2007

10. Cumulative Incidence No. of New Cases of Disease in a Time Period Number of persons in the population Expressed as a percentage: 2% of the 500,000 persons in City A developed diabetes in 2007

11. Lifetime Incidence (lifetime cumulative incidence) No. of New Cases of Disease in a Time Period Number of persons in the population Expressed as a percentage: 20% of the persons in City A developed diabetes in their lifetimes

12. Fixed Populations (study populations)

13. Incidence Density No. of New Cases of Disease in a Time Period Person-time at risk for the same time period

14. Calculation of Person-Time Example: 10 people followed for 3 years = 30 person-years or 360 person-months 3 people followed for 10 years = 30 person-years or 360-person months

15. Example Calculations Study subjects Incidence rate = 5/8 = 62.5/100 in 3 years Incidence density = 5/18 person-years at risk = 5/18 = 27.8/100 person-years Prevalence in year 2 = 3/6 = 50/100 Time (years) Patient died Patient lost to follow-up Red = disease Green= no disease

16. Incidence: Important Concepts Incidence is a measure of the risk of disease! As incidence goes up, so does risk The numerator is only NEW cases of disease The denominator must include only those persons at risk for developing the disease Time unit must be clearly specified All persons at risk must be followed the entire time period

17. Uterine Cancer Incidence Rates, Alameda County Years Rate Per 100,000

18. Differences Between Incidence and Prevalence Prevalence is only a snapshot of the population at a point or period in time - who has the disease and who does not Prevalence tells you nothing about how long patients have had the disease in the population Incidence includes only new cases or events among those at risk, and is a measure of risk of disease

19. Factors that Affect Prevalence: Prevalence rates are increased by: Immigration of cases or those with a high potential to become cases (elderly) Emigration of healthy individuals Increases in the duration of the disease ( earlier detection, better treatment, reduced fatality, etc.) Increases in incidence (occurrence of new cases)

20. Factors that Affect Prevalence: Prevalence rates are decreased by: Immigration of healthy persons Emigration of persons with disease Improved cure rates Increased death rates Decreases in incidence (occurrence of new cases) Shorter duration of the disease

21. Examples of Point and Period Prevalence, Incidence, and Cumulative Incidence Do you currently have asthma? Have you had asthma during the last year? Did you have your first episode of asthma this year? Have you ever had asthma? Point Prevalence Period Prevalence Incidence Cumulative or Lifetime Incidence

22. The Relationship Between Incidence and Prevalence Prevalence = Incidence x Duration

23. Low incidence and long duration Incidence Low Risk N = 4/yr. Duration Long 25 Yrs (25 X 4) Prevalence 100/1,000

24. High incidence and short duration Incidence High Risk N = 20/yr. Duration Short 3 Yrs (20 X 3) Prevalence 60/1,000

25. Prevalence of Coronary Heart Disease (CHD) Among 4,469 Persons, Framingham Study Rate Per 1,000 Age

26. Incidence of Coronary Heart Disease (CHD) Among 4495 Persons, Framingham Study Rate Per 1,000 Age

27. General Sources of Morbidity Statistics

28. Sources of Morbidity Information About Cancer Medical records Hospital Discharge Data Billing Records Medicare Cancer registries SEER (Surveillance, Epidemiology, and End Results ) SEER/Medicare linked data sets HEDIS (Health Plan Employer Data and Information Set)

29. SEER SEER – Surveillance, Epidemiology, and End Results Summary of cancer registry data from 4 states and 5 large metropolitan areas Represents 10% US population Used to estimate cancer rates for the nation

30. HEDIS Health Plan Employer Data and Information Set Established by National Committee for Quality Assurance Quality measures to compare health insurance plans

31. Incidence and Prevalence: Challenges with the Numerator Who has the disease? Diagnostic criteria can strongly impact rates Can be differences in case definitions Case definitions may change over time There may be changes in the quality of screening services Who Should Be Counted? Is case finding accurate and inclusive? How large is the effort to find all cases? Case finding by interview may yield inaccurate counts

32. PREVALENCE OF DEMENTIA BY DIFFERING CRITERIA PREVALENCE %

33. Number of Cases of Rape Number of Rapes 97,470 260,300 683,000 US Dept. of Justice Bureau of Justice Statistics (NCVS) 1995 National Women’s Study 1990 FBI Uniform Crime Report for 1995

34. AIDS CASES BY YEAR, 1984 - 1993 Years Thousands of Cases

35. SOURCES OF ERROR IN INTERVIEW SURVEYS Patient: Unaware of Disease Misunderstanding of Diagnosis Inaccurate Recall Refusal to Respond Altering of Response Interviewer : Interviewer Bias Selection of Subjects Lack of standardization of interview format

36. Limitations of Hospital Data Sources Admissions are selective Hospital records are not designed for research The denominator or population at risk is usually not known

37. Incidence and Prevalence: Problems with the Denominator Selective Undercounting – Who doesn’t get counted? Census Geographic Boundary changes Changes in Defined Populations Over Time

38. Surveillance A comprehensive system designed for the on-going detection and monitoring of the occurrence of health-related events or exposures in a target population

39. Key Features of Surveillance Continuous data collection and evaluation An identified target population A standard definition of the outcome of interest Emphasis on timeliness of collection and dissemination of results Use of data for purposes of disease investigation or control