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Chapter 5 Epidemiologic Principles and Methods Definition of Epidemiology • Epidemiology is defined as “the study of the distribution and determinants of disease frequency in human populations.” Step 1: Define the Disease • Death is easy to determine. – A death certificate states cause of death. • A blood test or stool culture is needed to verify a diagnosis of certain diseases. • Some diseases are hard to define. – EMS and SARS. • Sometimes a definition changes as more is learned. – AIDS • Other health outcomes include injuries and risk factors. Disease Frequency • Count the number of people with a disease and relate that to the population at risk (PAR). • PAR (denominator) may be the total population or exposed population, or one gender or age group. – PAR often comes from a census. • Two ways to measure frequency are: – Incidence, the number of new cases. – Prevalence, the number of existing cases. • Incidence is used for studying causes of disease. Disease Frequency, ctd. • Prevalence depends on incidence and prognosis. – If causes or risk factors increase, incidence and prevalence increase. – If ability to diagnose increases, incidence and prevalence appear to increase. • Prevalence rates are most useful in assessing the societal impact of a disease and planning for healthcare services. Distribution of Disease • Who – Sex, age, occupation, race, and economic status • When – Season, year (long-term trends), elapsed time since an exposure (epidemic curve). – Is crucial in tracking an outbreak of infectious diseases such as hepatitis and legionellosis. • Where – Neighborhood (e.g. clusters), latitude (climate), urban vs. rural, national variations. – Looks at comparisons of disease frequency in different countries, states, counties, or other geographical divisions. Determinants of Disease • Why is distribution as it is? • We can make inferences from distribution. • Epidemiologists usually speak of risk factors not causes. Human Population • Epidemiology studies human population, usually using observational rather than experimental methods. • Biomedical approach uses animal models to investigate the causes of disease. • Experiments conducted on animals can yield clear answers as to cause and effect. • For ethical reasons, experiments cannot usually be done on humans. Kinds of Epidemiologic Studies • Goal is to determine an association between an exposure and a disease or other health outcome. • Studies may be prospective or retrospective. • Intervention study • Cohort study • Case-control study Intervention Study • Epidemiologists do not perform the experiments. – Closest thing to an experiment. • Start with two groups: – Experimental group (gets the intervention or exposure) – Control group • Watch them over time and compare outcomes. • Experimenter chooses who is in which group. • Two groups should be as similar as possible so that intervention is the only difference. Intervention Study, ctd. • Randomized, double-blind, placebo control is ideal. • Pharmaceutical companies conduct many clinical trials for new drugs. • Physicians’ Health Study was a prevention study: – Aspirin to prevent heart disease. – Beta carotene to prevent cancer. • Field trial of polio vaccine in 1954 was randomized and double-blind. • Kingston–Newburgh study of fluoridation to prevent tooth decay was a community trial. Cohort Study • Are for situations when doing an intervention study would be unethical or too difficult. – Considered the next most accurate. • Choose a large number of healthy people, collect data on their exposures, and track outcomes over time. • The only difference from intervention is that people choose their own exposures. Cohort Study: Examples • Framingham Heart Study • Nurses’ Health Study • British study of physicians on smoking and lung cancer • Hammond–Horn study on smoking and lung cancer in the U.S. Case-Control Study • Faster and cheaper are the advantages. • This is the least accurate approach. • It is commonly done to follow up on a hypothesis generated by “shoe-leather” epidemiology. • Choose people who already have disease. • Choose a healthy control group of individuals, as similar as possible to cases. • Interview them all and ask for their previous exposures. • Estimate the strength of the association between exposure and disease by calculating an odds ratio. Discussion Question 1 • What is the difference between incidence and prevalence? • Why is incidence more useful in identifying the cause of a disease? • When is it most useful to use prevalence? • Give examples for each. Discussion Question 2 • Explain the interaction between incidence, prevalence, and prognosis. • Give examples. Discussion Question 3 • Why are the who, when, and where questions useful in determining the causes of disease? • Give examples. Discussion Question 4 • Explain the three major types of epidemiologic studies. • Which is most likely to yield a valid result? Why? • Which is likely to yield an answer in the shortest period of time? Why? Discussion Question 5 • Visit the National Institutes of Health website on clinical trials, www.clinicaltrials.gov. – Browse by condition. • How many clinical trials are going on right now? • Choose a condition and investigate what treatments are being tested for that condition. Discussion Question 6 • Visit the website of the Nurses’ Health Study, www.channing.harvard.edu/nhs. • Read the most recent annual newsletter. • What are the most recent findings of this cohort study?
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