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Epidemiology:
- study of distribution and determinants of health related states or events in specific populations and the application of this study to control health problems
- • Goal, essentially, is to limit undesirable health events in a community
- Epidemic:
- an unexpectedly large number of cases of an illness, specific health-related behavior, or other health related event in a particular population at a particular time and place
- Endemic disease:
- disease that occurs regularly in a population
- Epidemiologist
- : studies disease occurrence in defined populations
- Pandemic: outbreak of a disease over a wide geographical area, such as a continent
- • Example: avian influenza, which the books list as being the next possible agent to reach pandemic proportions.
- History of Epidemiology: 300 B.C., late 1600’s, 1849:
- : Hippocrates suggests link between disease and environment
- •Regression until late 1600’s; return to spiritual-causation belief
- •1849: John Snow studies epidemiology of cholera and extinguishes London outbreak, thereby limiting death and preserving health
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•Natality:
births / total population
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•Morbidity:
number of sick / total population at risk
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•Mortality:
- number deaths / total population at risk
- Based on population risk: those who are susceptible to a disease in a given population
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Incidence rate:
number of new health related cases of a disease / total number in population at risk
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The Importance of Rates Prime importance:
- •Number of events
- •Number of cases: the number of people who are sick
- •Number of deaths
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•Acute disease:
lasts less than 3 months
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•Chronic disease:
lasts longer than 3 months
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•Attack rate:
- incidence rate calculated for a specific population for a single disease outbreak expressed as a percentage
- oEx: number of people on a flight become sick, and it is suspected that the flight caused the illness. Attack rate could be calculated for the passengers to express who became ill.
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•Prevalence rates:
- all current cases / total population
- oUseful for chronic diseases: know how many are suffering
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•Crude rate:
denominator includes total population
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oCrude birthrate:
- # births in a year / mid year population
- +: easy to obtain, good for comparing populations
- -: misleading if populations differ in age/attributes
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•Age-adjusted rate:
rate used to make comparisons between groups over time when groups differ in age structures
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•Specific rates:
measures morbidity or mortality for particular populations or diseases
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Cause specific mortality rate (CSMR):
- death rate due to a particular disease
- Number of deaths by disease / total population
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Case fatality rate (CFR):
- percentage of cases of a disease that result in death
- # deaths in specified time / number of cases in same period
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Proportionate mortality ratio (PMR):
- percentage of overall mortality in a population that is attributable to particular cause
- # deaths due to specific disease / total number of deaths from all causes in same population in the same period of time
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notifiable diseases problems
- • sometimes only 35% of cases with diseases are reported
- • clinics may be too busy to report every case
- • patients can recover before treatment and may not seek care
- • reporting depends on disease severity
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most reliable indicator of a population health’s status
mortality statistics
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• Age adjusted rates show ...
- what death would be if no changes occurred in the age makeup of a population
- o they are a better indicator for examining changes in risk of death over a period of time
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-Life expectancy:
- average number of years a person from a specific cohort is projected to live from a given point of time
- • has generally increased in America
- -Years of potential life lost (YPLL):
- subtracting a person’s age at death from a predefined, standard age
- •age 75 often used as the standard
- operson dies at 59. 75-59 = 16 years
- vary depending on cause
- Accidents, cancer kill younger so higher YPLL
- -Disability adjusted Life years:
- measure for the burden of disease that takes into account premature death and loss of healthy life resulting from disability
- •mortality does not express burden of disease
- Ex: polio causes loss of healthy life but not reflected in death tables
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Health-adjusted life expectancy (HALE):
sometimes referred to has healthy life expectancy, number of years of healthy life expected, on average, in a given population
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-US Census: taken every 10 years; enumeration of the population living in the US
- •data: income, employment, family size, education, dwelling type, and many other social indicators
- •used for calculating disease and death rates for program planning
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Vital statistics:
- statistical summaries of vital records; records major life events
- •births, deaths, marriages, divorces
- birth: maternal and prenatal care, delivery
- deaths: rates, expectancy, leading causes, infant mortality
- Morbidity and Mortality Weekly Report
- -reported diseases are shown weekly
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-National Health Surveys:
- •health interviews of people
- •clinical tests
- •survey of places where care is received
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-NHANES: National health and nutrition examination survey
- •health/nutritional of the general US Population
- •based on PE, clinical and lab tests, and related procedures
- o most authoritative source of clinical info on American people
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-Behavioral Risk Factor Surveillance System:
state-based telephone survey to ascertain prevalence of high risk behavior, like smoking or drinking
- -Youth Risk Behavior Survey (YRBS): six categories of risks in youth
- •behaviors that contribute to unintentional violence and injuries
- •tobacco use
- •alcohol and drug use
- •STIS and pregnancy
- •HIV
- •Unhealthy dietary habits and physical inactivity
- -National Health Care Survey (NHCS):
- nine different national surveys that gather info on nation’s healthcare system
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-Descriptive studies:
- describes disease with respect to person, place, and time (the extent, essentially)
- First studies carried out on diseases!! Valuable info on detection patterns.
- Who: head count, as well as age, sex, race, marital status, etc.
- When: categorized by time of occurrence
- Summarized by year, for example
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Common source epidemic curve:
graphic display of disease where each case can be traced to a single source of exposure Used to calculate incubation period
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incubation period:
time between exposure and onset of symptoms
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Point source:
exposure occurs at a single point in time
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Continuous source:
- exposure is intermittent
- Need to discover these to prevent future exposure
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Propagated man parts: epidemic curve depicting distribution of cases traceable to multiple sources of exposure
- Primary cases appear at one end, secondary cases after primary, and tertiary further along the graph
- Where: residential addresses, travel history are recorded
- Restaurants, shopping, etc.
- •Limitations of descriptive studies: not applicable to outbreaks elsewhere and cannot provide info on disease trends.
- -Analytic studies:
- test hypotheses about relationships between health problems and possible risk factors
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• Risk factors
- • increase probability of disease
- o Ex. Does smoking increase risk of lung cancer?
- • Two different types of analytic studies exist:
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Observational studies:
- investigates natural course of disease
- Observational studies can be divided into:
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Cross sectional:
info about exposure and disease are collected at the same time
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Case/control:
- seeks to compare diagnosed with a disease and those who are not for prior exposure to specific risks
- • Familial, behavioral links
- • Don’t prove causation alone, but indicate future direction of question
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o Cohort studies:
- cohort is classified by exposure to one or more risk factors and observed to determine the rates at which the disease develops in each class
- Can be prospective (observing a group to see if cigarette smoking leads to disease) or retrospectively (exposure and outcome has already happened)
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o Case controls cannot calculate the
o rate of development of a disease, as the patients already have it
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Odds ratio:
probability about association between risk factors and developing a disease
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Cohort studies can
calculate incidence rate
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Relative risk:
statement of relationship between risk factor and disease (more definitive)
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Cohort disadvantages:
expensive, not practical for rare cases, and take many years to complete
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Experimental studies:
- carried out to identify cause of a disease or determine effectiveness of an intervention
- Use of control and experimental groups, placebo, etc.
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Randomization:
assigning subjects to treatments in a non-organized manner
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Blinding:
subject remain uninformed and unaware of groups
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