This document defines epidemiology and describes key epidemiological principles and study methods. It defines epidemiology as the study of disease distribution and determinants in human populations. It describes the steps of defining a disease, measuring its frequency through incidence and prevalence, and examining its distribution based on who, when and where is affected. It also discusses determining disease determinants and outlines three major epidemiological study designs: intervention studies, cohort studies, and case-control studies. Intervention studies most closely mimic experiments but cohort studies are more accurate when experiments cannot be done. Case-control studies are fastest but least accurate.

1. Copyright © 2021 by Jones & Bartlett Learning, LLC an Ascend Learning Company. www.jblearning.com.
CHAPTER 5
Epidemiologic
Principles and
Methods

2. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
Definition of Epidemiology
 Epidemiology is defined as “the study of the distribution and determinants of
disease frequency in human populations.”

3. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

4. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

5. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

6. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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

7. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
Determinants of Disease
 Why is distribution as it is?
 We can make inferences from distribution.
 Epidemiologists usually speak of risk factors not causes.

8. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

9. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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

10. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

11. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

12. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

13. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

14. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

15. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

16. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
Discussion Question 2
 Explain the interaction between incidence, prevalence, and prognosis.
 Give examples.

17. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
Discussion Question 3
 Why are the who, when, and where questions useful in determining the causes
of disease?
 Give examples.

18. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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?

19. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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.

20. Copyright
©
2021
by
Jones
&
Bartlett
Learning,
LLC
an
Ascend
Learning
Company.
www.jblearning.com
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?