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

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. The formula for the mortality of a defined population, over a specified period of time, is:

Deaths occurring during a given time period

[Image: divided by]

Size of the population among which
the deaths occurred

× 10 n

 

When mortality rates are based on vital statistics (e.g., counts of death certificates), the denominator most commonly used is the size of the population at the middle of the time period. In the United States, values of 1,000 and 100,000 are both used for 10n for most types of mortality rates. Table 3.4 summarizes the formulas of frequently used mortality measures.

Table 3.4 Frequently Used Measures of Mortality

Measure

Numerator

Denominator

10n

Crude death rate

Total number of deaths during a given time interval

Mid-interval population

1,000 or
100,000

Cause-specific death rate

Number of deaths assigned to a specific cause during a given time interval

Mid-interval population

100,000

Proportionate mortality

Number of deaths assigned to a specific cause during a given time interval

Total number of deaths from all causes during the same time interval

100 or 1,000

Death-to-case ratio

Number of deaths assigned to a specific cause during a given time interval

Number of new cases of same disease reported during the same time interval

100

Neonatal mortality rate

Number of deaths among children
< 28 days of age during a given time interval

Number of live births during the same time interval

1,000

Postneonatal mortality rate

Number of deaths among children 28–364 days of age during a given time interval

Number of live births during the same time interval

1,000

Infant mortality rate

Number of deaths among children
< 1 year of age during a given time interval

Number of live births during the same time interval

1,000

Maternal mortality rate

Number of deaths assigned to pregnancy-related causes during a given time interval

Number of live births during the same time interval

100,000

Crude mortality rate (crude death rate)

The crude mortality rate is the mortality rate from all causes of death for a population. In the United States in 2003, a total of 2,419,921 deaths occurred. The estimated population was 290,809,777. The crude mortality rate in 2003 was, therefore, (2,419,921 ⁄ 290,809,777) × 100,000, or 832.1 deaths per 100,000 population.(8)

Cause-specific mortality rate

The cause-specific mortality rate is the mortality rate from a specified cause for a population. The numerator is the number of deaths attributed to a specific cause. The denominator remains the size of the population at the midpoint of the time period. The fraction is usually expressed per 100,000 population. In the United States in 2003, a total of 108,256 deaths were attributed to accidents (unintentional injuries), yielding a cause-specific mortality rate of 37.2 per 100,000 population.(8)

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