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  • 1. Epidemiology: The Study of Disease,Injury, and Death in the Community Chapter 3
  • 2. Introduction• Epidemiology = population medicine• Epidemiologists concerned with course of disease in a population• Collect information about disease status of a community • How many people are sick? Who is sick? When did they become sick? Where do they live?• Data can be used to prevent disease outbreaks or determine effectiveness of prevention effort
  • 3. Definitions• Epidemiology: study of distribution and determinants of health-related states or events in specified populations• Epidemic: unexpectedly large number of cases of an illness, specific health-related behavior or event, in a particular population• Endemic: disease that occurs regularly in a population as a matter of course• Pandemic: outbreak over wide geographic area
  • 4. The Importance of Rates• Rates allow for comparison of outbreaks at different times or in different places• Cases: people afflicted (those who are sick)• Rates: number of events in a given population over a given period of time or given point in time • Natality (birth), morbidity (sickness), mortality or fatality (death) rates• Population at Risk: those susceptible to particular disease or condition
  • 5. Morbidity Rates• Incidence rate: number of new health-related events or cases of a disease in a population exposed to that risk during a particular period of time, divided by total # in same population• Prevalence rate: number of new and old cases in a given period of time, divided by total # in that population• Attack rate: incidence rate calculated for a particular population for a single disease outbreak; expressed as a percentage
  • 6. Incidence, Prevalence, and Attack Rates• Incidence rates important to study of acute diseases • Acute disease: lasts three months or less• Prevalence rates useful for study of chronic diseases • Chronic disease: last longer than three months• Incidence and prevalence rates expressed as crude or specific
  • 7. Incidence, Prevalence, and Attack Rates
  • 8. Crude and Age-Adjusted Rates• Crude rates: denominator includes the total population • Crude birth rate: # of live births in given year, divided by midyear population • Crude death rate: # of deaths in given year from all causes, divided by midyear population• Age-adjusted rates: used to make comparisons of relative risks across groups and over time when groups differ in age structure
  • 9. Crude Rates
  • 10. Crude and Age-Adjusted Rates
  • 11. Specific Rates• Measure morbidity and mortality for particular populations or diseases• Case fatality rate: percentage of cases of a particular disease that result in death• Proportionate mortality ratio: percentage of overall mortality in a population that is attributable to a particular cause
  • 12. Important Rates in Epidemiology
  • 13. Reporting of Birth, Deaths, and Diseases• Physicians, clinics, and hospitals required to report births, deaths, and notifiable diseases • Notifiable diseases: infectious diseases in which health officials request or require reporting • Can become epidemics • Health officials maintain weekly records• Various challenges to maintaining accurate data
  • 14. Notifiable Disease Scheme
  • 15. Standardized Measurements of Health Status of Populations• Mortality statistics most reliable measure of population health status • Easier to track death than illness• Life expectancy: average number of years a person from a specific cohort is projected to live from a given point in time• Years of potential life lost (YPLL): number of years lost when death occurs before one’s life expectancy
  • 16. Life Expectancy Figures
  • 17. Years of Potential Life Lost• Subtract person’s age at death from his or her life expectancy• Difficult to determine because life expectancy changes at different ages• Weighs death of young person as counting more than death of old
  • 18. Sources of Standardized Data• Various valid sources have specific value and usefulness to those in public health • U.S. Census • Taken every 10 years • Enumeration of the population • More complex now • Gathers data on race, age, income, employment, education, and other social indicators
  • 19. Sources of Standardized Data (ctd)• Statistical Abstract of the United States • Book published annually by Bureau of Census • Summary of statistics on social, political, and economic organization of the United States• Monthly Vital Statistics Report • Vital statistics are summaries of records of major life events: birth, death, marriage, divorce • Published by National Center for Health Statistics under the CDC • Also calculates death rates by race and age
  • 20. Sources of Standardized Data (ctd)• Morbidity and Mortality Weekly Report (MMWR) • Prepared by CDC from state health department reports • Reports morbidity and mortality data by state and region of U.S. • Reports outbreaks of disease, environmental hazards, unusual cases, or other public health problems
  • 21. National Health Surveys• National Health Survey Act of 1956 authorized continuing survey of amount, distribution, and effects of illness and disability in the U.S.• Three types of surveys • Health interviews of people • Clinical tests, measurements, and physical examinations • Surveys of places where people receive medical care
  • 22. Some National Health Surveys• National Health Interview Survey (NHIS) • Conducted by NCHS • Questions respondents about their health• National Health and Nutrition Examination Survey (NHANES) • Assesses health and nutrition status through mobile examination center• Behavioral Risk Factor Surveillance System (BRFSS); National Health Care Survey (NHCS)
  • 23. Epidemiological Studies• Investigations carried out when disease or death occurs in unexpected or unacceptable numbers• Descriptive studies • Describe epidemics with respect to person, place, and time• Analytic studies • Aimed at testing hypotheses
  • 24. Descriptive Studies• Who? • Case count, followed by who is ill (children, men, women, race, etc.)• When? • Time of onset for each case • Epidemic curves created• Where? • Determine residential address and travel history
  • 25. Epidemic Curves• Graphic display of the cases of disease according to the time or date of onset of symptoms• Secular, seasonal, and single epidemic curves• Single epidemic curves • Point source epidemic curve • Propagated epidemic curve
  • 26. Secular Epidemic Curve
  • 27. Seasonal Epidemic Curve
  • 28. Point Source Epidemic Curve
  • 29. Propagated Epidemic Curve
  • 30. Analytic Studies• Test hypotheses about relationships between health problems and possible risk factors • Observational studies: investigator observes natural course of events, noting exposed vs. unexposed and disease development • Case/control studies • Cohort studies • Experimental studies: investigator allocates exposure and follows development of disease
  • 31. Case/Control Studies• Case/Control • Compares those with disease to those without but with similar background and/or with prior exposure to certain risk factors • Aimed at identifying factors more common in case than control group
  • 32. Cohort Studies• Cohort is classified by exposure to one or more risk factors and observed to determine rate of disease development • Cohort: group of people who share important demographic characteristic• Odds ratio• Relative risk
  • 33. Experimental Studies• Carried out to identify cause of disease or determine effectiveness of vaccine, drug, or procedure • Control for variables • Control groups • Randomization • Blinding • Placebo: blank treatment
  • 34. Criteria of Causation• Questions exposure causing development of disease• Criteria • Strength • Consistency • Specificity • Temporality • Biological plausibility
  • 35. Discussion Questions• How can data collection for notifiable diseases be improved?• Why is tracking vital statistics so important?• How does calculating Years of Potential Life Lost change the way we think about community health efforts?