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Statistical Literacy: Epidemiology

Key concepts in epidemiology for APRNs

Common Measures of Mortality (death)

Crude Mortality (All-Cause) is the total number of deaths from all causes, sometimes referred to as All-Cause Mortality.

Maternal Mortality is when a person dies during pregnancy or up to 42 days after the end of pregnancy* from health problems related to pregnancy. Also called Maternal Death in some resources (National Center for Chronic Disease Prevention and Health Promotion, 2020).
*Please be careful not to confuse this with pregnancy-related mortality which is defined as when a person dies during pregnancy or within 1 year after the end of her pregnancy from health problems related to pregnancy. This measure of natality is used by the WHO (World Health Organization, 2015).

Infant Mortality is defined as  the death of an infant before their first birthday (after a live birth) (Centers for Disease Control and Prevention, 2021).

Common Measures of Morbidity (illness)

Incidence describes only the number of new cases that occur during that time period of interest. Thus, incidence statistics can inform you of emerging problems such as an increase in the number of cases of a pre-existing disease or the emergence of a new disease or type of injury.

EXAMPLE

The incidence rate is the number of new cases of the health state or event of interest, at a specified point in time. For example, the number of new diagnoses of HIV/AIDS in the calendar year 2016 for a particular geographic region.


Prevalence describes the total number of cases of the health state of interest (disease, injury, risk behavior). This means that all cases, new and pre-existing, are counted for the specified time of interest.  Prevalence is a measure that helps us discuss the burden of disease in a population so we can plan for services that might be needed.

EXAMPLE


The prevalence rate is the number of living cases with the health state or event of interest, at a specified point in time.  For example, the number of people living with HIV/AIDS in the calendar year 2016 for a particular geographic region.


Natality refers to births. Common measures of birth include:

  • Crude birth rate: the number of live births during a specified time interval among the total population
  • Crude fertility rate: the number of live births during a specified time interval among the number of women ages 15–44 years

Different Ways to Express Measures

Counts: count, or frequency, is the number of cases. Counts are good, but they do not provide much context. Epidemiologists use counts to discuss disease status in a population.  However, they more often use rates and percentages.

Rates provide more information. In epidemiology, a rate is a measure of the frequency with which an event occurs in a defined population over a specified time. Rates tell us the count of disease in a population of interest over time. The focus of the rates could be the entire population (crude rate) or could be out of a population of interest (adjusted rate). Natality rate is an excellent example of a rate, as it is births/population in a certain time (AKA Birthrate!). Often, in epidemiology, disease rates are described out of 100,000 population because it is a way to conceptualize and compare the numbers more easily.

Does it matter if you choose to look at crude rates or adjusted rates? The answer is yes if everyone in the general population is not affected equally by the condition, or if the populations are not comparable.
For example, the 1980 crude cancer mortality rate was 183.8 per 100,000 per year. Does this tell the story for children? No. The adjusted cancer mortality rate for children less than five years old was 4.2 per 100,000 per year. This makes sense when we think about there being a big difference in the number and types of cancers that affect children versus elderly people.

Percentages are a way to compare numbers out of the same denominator, 100. If you want to calculate a percentage, take the number of cases/numbers in the population then multiply by 100.  If you have a picnic with 15 people and four get sick, the way to calculate % sick is 4/15~.267. Then multiply by 100. This means 26.7% of the people at the picnic got sick.

Proportions are a way to evaluate how a population is affected by the disease or condition. To calculate proportions only include cases that occur in a defined population as your numerator and use the defined population as your denominator. Basically you are looking at cases in a defined population. Adjusted rate (above) is a way of examining proportions.

References

Centers for Disease Control and Prevention. (2021). Infant mortality. https://www.cdc.gov/reproductivehealth/maternalinfanthealth/infantmortality.htm

National Center for Chronic Disease Prevention and Health Promotion. (2020). Maternal mortality. https://www.cdc.gov/reproductivehealth/maternal-mortality/index.html

World Health Organization. (2015). 2015 global reference list of 100 core health indicators. https://apps.who.int/iris/bitstream/handle/10665/173589/WHO_HIS_HSI_2015.3_eng.pdf;jsessionid=CCD89F6CF2C650EBE55AD31F7DE5F370?sequence=1