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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
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
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
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
• 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
Incidence, Prevalence, and Attack Rates
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
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
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
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
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
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
• 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
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
Sources of Standardized Data (ctd)
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
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)
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
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
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
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
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
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
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
Criteria of Causation
• Questions exposure causing development of disease
• Criteria• Strength
• Consistency
• Specificity
• Temporality
• Biological plausibility