Introduction to Epidemiologic Methods

Welcome to Module 1. Understanding how and why diseases are distributed in populations and what factors are associated with disease relies on a set of epidemiologic methods that can be applied flexibly to many different settings. This unit introduces the main concepts in epidemiology and reviews the methodological approaches to measuring diseases in populations and assessing relationships between exposures and diseases.

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If you have trouble watching the lectures due to a slow Internet connection, try downloading the lecture or the transcript listed below each video. Right click the link and select "Save Link as" to download.

Learning Objectives

After completing this module, you will be able to:

  • Describe the main concepts in epidemiology.
  • Describe the methodological approaches to measuring diseases in populations and assessing relationships between exposures and diseases.
  • Give an example of a disease that is distributed unevenly in a population and what the distribution might tell you about the cases of disease.
  • Define prevalence and incidence and describe the steps to measure each in a typical epidemiologic study.
  • Explain how to compare disease risk between two groups and how to interpret these comparisons.

Video: Intro to Epi: Basic Terminology Part 1 (12 minutes)

In this lecture, you will learn some basic epidemiological terminology from Dr. Brandon Guthrie.

Downloads: Video (17 MB), Audio (6 MB), Transcript

Video: Intro to Epi: Basic Terminology Part 2 (12 minutes)

This lecture will continue with basic epidemiologic terms.

Downloads: Video (10 MB), Audio (4 MB), Transcript

Video: Descriptive Epidemiology (6 minutes)

Dr. Brandon Guthrie discusses descriptive epidemiology and the types of data collected in descriptive epidemiologic studies in this lecture.

Downloads: Video (7 MB), Audio (3 MB), Transcript

Video: Measuring Burden of Disease (10 minutes)

Watch this lecture to learn about the burden of disease. Dr. Brandon Guthrie will talk specifically about prevalence and incidence.

Downloads: Video (11 MB), Audio (5 MB), Transcript

Activity: Cumulative Incidence, Incidence Rate, & Prevalence (15 minutes)

Test how well you understand the differences between cumulative incidence, incidence rate, and prevalence by taking this short, ungraded quiz. Although this activity is optional, it will help reinforce these concepts.

The information in the questions below is based on a hypothetical studies. For each of the fractions, select whether it is an incidence proportion, incidence rate, or prevalence.

Video: Measuring Excess Risk (6 minutes)

Dr. Brandon Guthrie talks about measuring excess risk in this lecture.

Downloads: Video (7 MB), Audio (3 MB), Transcript

Activity: Measures of Excess Risk (15 minutes)

Test how well you understand relative risk, odds ratios, and other measures of excess risk by taking this short, ungraded quiz. It may be helpful to refer to the resource document that lists the various equations covered in this module. Although this activity is optional, it will help reinforce these concepts.

Use this 2x2 table to answer the first two questions.

Table 1: Incidence of Mycobacterium Tuberculosis Infection Among Congregated, HIV-Infected Prison Inmates by Dormitory Wing, 1999

Developed TB?
Yes No Total
East wing a = 28 b = 129 H1 = 157
West wing c = 4 d = 133 H0 = 137
Total 32 262 T = 294

Adapted from: McLaughlin SI, Spradling P, Drociuk D, Ridzon R, Pozsik CJ, Onorato I. Extensive transmission of Mycobacterium tuberculosis among congregated, HIV-infected prison inmates in South Carolina, United States. Int J Tuberc Lung Dis 2003;7:665–672.


Reading: Using Counts & Rates (30 minutes)

The following reading covers count, rates, and descriptive epidemiology. It will reinforce some of what has been covered in the lectures.

Using Counts & Rates, Descriptive Epidemiology. In Introduction to Epidemiology. Pp 1.30-1.44.

Reading: Module Summary (5 minutes)

Module 1 introduced you to epidemiology and some common epidemiological terms. Epidemiology is the study of the distribution and factors or causes of health-related states or events in specified populations and the application of this knowledge to address health problems. An epidemic, or outbreak, is when a disease occurs at a level that is greater than what is expected in a specific population in a defined geographical area within a defined time period. A pandemic is when an epidemic spreads worldwide, across multiple countries or continents, and affects a high proportion of the global population. A case definition is a specific set of criteria for classifying individuals as having a disease or condition. Unless we know whether something is defined as a case or not a case at a given time point, we'll be making very different decisions in terms of including them in our assessments.

A disease event is used to describe diseases or outcomes that happen essentially instantaneously. They can be recurrent, meaning that they can happen multiple times, or non-recurrent (happening only once). Disease states are used to describe periods of time when an individual is in a susceptible, diseased, or resistant or in an immune state. Diseases that have some duration are described as diseased states rather than disease events.

Susceptibility is used to define the population that is “at-risk” for the disease of interest.

A fundamental concept in measuring disease or outcome occurrence is observation time. This is the time that an individual, participant, or patient is being observed for the presence or onset of disease or the outcome of interest.

Defined populations are a group of individuals that can be defined by certain shared characteristics. Populations can be closed, meaning membership in the population does not change over the period of observation, or open, meaning the group can gain or lose members during the period of observation.

This module also covered descriptive epidemiology, which describes a disease in a population by person, place, and time. These characteristics provide important clues about the underlying cause of disease or the source of disease in case of outbreaks. Descriptive statistics around people help us identify characteristics of people with disease. Place data provide information on the geographic extent of the disease. When we think about time data, we need to consider different ways, such as calendar time, age, and time, since an event. We also need to look at trends or long-term variation as well as short-term seasonal variations.

Next you learned about measuring the burden of disease. Prevalence is how common a disease is at a specified time. Prevalence is also called point-prevalence because it determines the disease occurrence in a snapshot of time. In contrast to prevalence, which measures the frequency of existing cases, incidence measures how rapidly people develop the disease over time. Cumulative incidence is defined as the number of people who develop the disease among the number of people who were initially at risk. For a specified population of people at risk of disease who may be followed for different amounts of time for the onset of disease, we can calculate the incidence rate. There are other types of incidence; for example, case fatality, which measures the risk that if a person gets a disease, they will die of that disease. Cumulative mortality is the burden of fatal disease on a population as a whole. Proportional mortality is the burden of mortality due to a specific disease out of all causes of death.

Finally, you learned about measuring excess risk. The relative risk measures the strength of an association between the exposure and the outcome. The odds ratio is the ratio of the “odds” in contrast to relative risk, which is the ratio of “risks”. Odds ratio approximates the relative risk when outcome is rare. The risk difference is the difference in the risk between exposed and non-exposed persons. Attributable risk is difference in incidence between an exposed and unexposed group when we assume that the exposure actually causes the difference in risk between the exposed and unexposed. Population attributable risk is the incidence of disease in the total population that is due to the exposure.

We have provided a resource document listing all of the equations.

Quiz (15 minutes)

Go to Quiz

*Please note that there is no access to the quiz in this sample course.

Optional Resources

Common Epidemiological Equations

Freely Available Epidemiologic Software, Calculators, and Useful Websites.

Definition of Epidemiology, Historical Evolution of Epidemiology, Uses, Core Epi Functions. In Introduction to Epidemiology. Pp 1.1-1.19.

Measures of Disease

Measures of Risk. In Introduction to Epidemiology. Pp. 3.1-3.32. 

Measures of Association. In Introduction to Epidemiology. Pp. 38-45.