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  • 1. Epidemiology: The Study of Disease,Injury, and Death in theCommunityChapter 3

2. Introduction Epidemiology = population medicine Epidemiologists concerned with course ofdisease in a population Collect information about disease status of acommunity How many people are sick? Who is sick?When did they become sick? Where do theylive? Data can be used to prevent disease outbreaksor determine effectiveness of prevention effort 3. Definitions Epidemiology: study of distribution anddeterminants of health-related states or events inspecified populations Epidemic: unexpectedly large number of cases ofan illness, specific health-related behavior orevent, in a particular population Endemic: disease that occurs regularly in apopulation as a matter of course Pandemic: outbreak over wide geographic area 4. The Importance of Rates Rates allow for comparison of outbreaks atdifferent times or in different places Cases: people afflicted (those who are sick) Rates: number of events in a given populationover 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 particulardisease or condition 5. Morbidity Rates Incidence rate: number of new health-relatedevents or cases of a disease in a populationexposed to that risk during a particular period oftime, divided by total # in same population Prevalence rate: number of new and old cases ina given period of time, divided by total # in thatpopulation Attack rate: incidence rate calculated for aparticular population for a single diseaseoutbreak; expressed as a percentage 6. Incidence, Prevalence, and Attack Rates Incidence rates important to study of acutediseases Acute disease: lasts three months or less Prevalence rates useful for study of chronicdiseases Chronic disease: last longer than three months Incidence and prevalence rates expressed ascrude or specific 7. Incidence, Prevalence, and Attack Rates 8. Crude and Age-Adjusted Rates Crude rates: denominator includes the totalpopulation Crude birth rate: # of live births in given year,divided by midyear population Crude death rate: # of deaths in given yearfrom all causes, divided by midyear population Age-adjusted rates: used to make comparisonsof relative risks across groups and over timewhen groups differ in age structure 9. Crude Rates 10. Crude and Age-Adjusted Rates 11. Specific Rates Measure morbidity and mortality for particularpopulations or diseases Case fatality rate: percentage of cases of aparticular disease that result in death Proportionate mortality ratio: percentage ofoverall mortality in a population that isattributable to a particular cause 12. Important Rates in Epidemiology 13. Reporting of Birth, Deaths, and Diseases Physicians, clinics, and hospitals required toreport births, deaths, and notifiable diseases Notifiable diseases: infectious diseases inwhich health officials request or requirereporting Can become epidemics Health officials maintain weekly records Various challenges to maintaining accuratedata 14. Notifiable Disease Scheme 15. Standardized Measurements of Health Status ofPopulations Mortality statistics most reliable measure ofpopulation health status Easier to track death than illness Life expectancy: average number of years aperson from a specific cohort is projected tolive from a given point in time Years of potential life lost (YPLL): number ofyears lost when death occurs before ones lifeexpectancy 16. Life Expectancy Figures 17. Years of Potential Life Lost Subtract persons age at death from his or herlife expectancy Difficult to determine because life expectancychanges at different ages Weighs death of young person as countingmore than death of old 18. Sources of Standardized Data Various valid sources have specific value andusefulness 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 socialindicators 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 departmentreports Reports morbidity and mortality data by stateand region of U.S. Reports outbreaks of disease, environmentalhazards, unusual cases, or other public healthproblems 21. National Health Surveys National Health Survey Act of 1956authorized continuing survey of amount,distribution, and effects of illness anddisability in the U.S. Three types of surveys Health interviews of people Clinical tests, measurements, and physicalexaminations Surveys of places where people receivemedical care 22. Some National Health Surveys National Health Interview Survey (NHIS) Conducted by NCHS Questions respondents about their health National Health and Nutrition ExaminationSurvey (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 ordeath occurs in unexpected or unacceptablenumbers 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 diseaseaccording to the time or date of onset ofsymptoms 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 betweenhealth problems and possible risk factors Observational studies: investigator observesnatural course of events, noting exposed vs.unexposed and disease development Case/control studies Cohort studies Experimental studies: investigator allocatesexposure and follows development of disease 31. Case/Control Studies Case/Control Compares those with disease to those withoutbut with similar background and/or with priorexposure 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 ormore risk factors and observed to determinerate of disease development Cohort: group of people who share importantdemographic characteristic Odds ratio Relative risk 33. Experimental Studies Carried out to identify cause of disease ordetermine effectiveness of vaccine, drug, orprocedure Control for variables Control groups Randomization Blinding Placebo: blank treatment 34. Criteria of Causation Questions exposure causing development ofdisease Criteria Strength Consistency Specificity Temporality Biological plausibility 35. Discussion Questions How can data collection for notifiable diseasesbe improved? Why is tracking vital statistics so important? How does calculating Years of Potential LifeLost change the way we think aboutcommunity health efforts?