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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 (cont.)

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.

Mortality rates are used to measure frequency for diseases that are often fatal.

Distribution of Disease

Who:

Sex, age, occupation, race, and economic status

When:

Looks for disease frequency over time: 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 (cont.)

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 shoeleather 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?