Course Content
What Is Epidemiology?
Epidemiology is the branch of medical science that investigates all the factors that determine the presence or absence of diseases and disorders. Epidemiological research helps us to understand how many people have a disease or disorder, if those numbers are changing, and how the disorder affects our society and our economy.
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What Is Epidemiology
Planning and conducting a survey
Epidemiological surveys use various study designs and range widely in size. At one extreme a case-control investigation may include fewer than 50 subjects, while at the other, some large longitudinal studies follow up many thousands of people for several decades.
Frequency Measures
A measure of central location provides a single value that summarizes an entire distribution of data. In contrast, a frequency measure characterizes only part of the distribution.
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Frequency Measures
Morbidity Frequency Measures
Morbidity has been defined as any departure, subjective or objective, from a state of physiological or psychological well-being. In practice, morbidity encompasses disease, injury, and disability.
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Morbidity Frequency Measures
Mortality Frequency Measures Mortality rate
A mortality rate is a measure of the frequency of occurrence of death in a defined population during a specified interval. Morbidity and mortality measures are often the same mathematically; it’s just a matter of what you choose to measure, illness or death.
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Mortality Frequency Measures
Measures of Public Health Impact
A measure of public health impact is used to place the association between an exposure and an outcome into a meaningful public health context. Whereas a measure of association quantifies the relationship between exposure and disease, and thus begins to provide insight into causal relationships, measures of public health impact reflect the burden that an exposure contributes to the frequency of disease in the population.
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Measures of Public Health Impact
Measures of Association
The key to epidemiologic analysis is comparison. Occasionally you might observe an incidence rate among a population that seems high and wonder whether it is actually higher than what should be expected based on, say, the incidence rates in other communities.
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Measures of Association
Reading epidemiological reports
Epidemiological methods are widely applied in medical research, and even doctors who do not themselves carry out surveys will find that their clinical practice is influenced by epidemiological observations.
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Reading epidemiological reports
Epidemiology: The Basic Science of Public Health
About Lesson

Morbidity has been defined as any departure, subjective or objective, from a state of physiological or psychological well-being. In practice, morbidity encompasses disease, injury, and disability. In addition, although for this lesson the term refers to the number of persons who are ill, it can also be used to describe the periods of illness that these persons experienced, or the duration of these illnesses.(4)

Measures of morbidity frequency characterize the number of persons in a population who become ill (incidence) or are ill at a given time (prevalence). Commonly used measures are listed in Table 3.3.

Table 3.3 Frequently Used Measures of Morbidity

Measure

Numerator

Denominator

Incidence proportion
(or attack rate or risk)

Number of new cases of disease during specified time interval

Population at start of time interval

Secondary attack rate

Number of new cases among contacts

Total number of contacts

Incidence rate
(or person-time rate)

Number of new cases of disease during specified time interval

Summed person-years of observation or average population during time interval

Point prevalence

Number of current cases (new and preexisting) at a specified point in time

Population at the same specified point in time

Period prevalence

Number of current cases (new and preexisting) over a specified period of time

Average or mid-interval population

Incidence refers to the occurrence of new cases of disease or injury in a population over a specified period of time. Although some epidemiologists use incidence to mean the number of new cases in a community, others use incidence to mean the number of new cases per unit of population.

Two types of incidence are commonly used — incidence proportion and incidence rate.

Incidence proportion or risk Definition of incidence proportion

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Incidence proportion is the proportion of an initially disease-free population that develops disease, becomes injured, or dies during a specified (usually limited) period of time. Synonyms include attack rate, risk, probability of getting disease, and cumulative incidence. Incidence proportion is a proportion because the persons in the numerator, those who develop disease, are all included in the denominator (the entire population).

Method for calculating incidence proportion (risk)

Number of new cases of disease or injury during
specified period

[Image: divided by]

Size of population at start of period

EXAMPLES: Calculating Incidence Proportion (Risk)

Example A: In the study of diabetics, 100 of the 189 diabetic men died during the 13-year follow-up period. Calculate the risk of death for these men.

Numerator = 100 deaths among the diabetic men
Denominator = 189 diabetic men
10n = 102 = 100

Risk = (100 ⁄ 189) × 100 = 52.9%

Example B: In an outbreak of gastroenteritis among attendees of a corporate picnic, 99 persons ate potato salad, 30 of whom developed gastroenteritis. Calculate the risk of illness among persons who ate potato salad.

Numerator = 30 persons who ate potato salad and developed gastroenteritis
Denominator = 99 persons who ate potato salad
10n = 102 = 100

Risk = “Food-specific attack rate” = (30 ⁄ 99) × 100 = 0.303 × 100 = 30.3%

Properties and uses of incidence proportions

  • Incidence proportion is a measure of the risk of disease or the probability of developing the disease during the specified period. As a measure of incidence, it includes only new cases of disease in the numerator. The denominator is the number of persons in the population at the start of the observation period. Because all of the persons with new cases of disease (numerator) are also represented in the denominator, a risk is also a proportion.
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