Risks and Rates

Risks( synonyms: incidence proportion, attack rate) are expressed as percentages.

In a population, the risk of a disease measures the proportion of people that develops this disease between two specified points in time (T0 and T1). It is calculated as the number of persons developing the disease during the observation period divided by the number of persons present at the beginning of the observation period. The denominator represents the population at risk of developing the disease at the beginning of the observation period. The time spent by each individual in the observation period is not considered.

A risk is a probability. It will range from 0 to 1, or 0% to 100% if expressed as a percentage. The risk of disease is therefore also called incidence proportion.

An example in which 50 out of 200 nursing home residents developed gastroenteritis between 12 and 20 May 2005, the risk of gastroenteritis is 50/200 = 0.25 or 25%.

In intervention-epidemiology, risk is used for short follow-up periods, like outbreaks during which all individuals are assumed to be followed for the same period.

In outbreaks, the term risk is frequently replaced by attack rate, which is also expressed as a percentage. Even though widely accepted in the epidemiological community, the term “attack rate” is technically incorrect, as an attack rate is not a rate but a risk. In outbreaks, a food-specific attack rate measures how many people who ate a certain type of food (or had another exposure) became ill (else "were attacked by that food").

The measurement of risk shows the probability of developing an illness during a specific period of time. However, without knowledge of the time period, it is impossible to interpret a risk . A risk of death of 2% will imply different meanings if it expresses the risk of death during one month, one year, or a 30-year follow-up.

Rates Rates are a measure of the occurrence of a phenomenon . A rate is calculated as the number of events that have occurred, divided by the total time experienced by the population under observation, usually expressed in person-years. Rates are usually multiplied by a power of 10 to convert the rate into a decimal or whole number, which is easy to interpret.

In a study, it is possible that each person experiences the same event more than one time.

Incidence rates are a subset of rates in which we are interested in quantifying new events ("new cases" of disease)

The term rate is sometimes used incorrectly in epidemiology. For example, to replace the term risk (as in attack rates and case fatality rates).

Credits

FEM Editor 2007

• Sabrina Bacci

Original Authors

• Alain Moren
• Marta Valenciano
• Arnold Bosman

FEM Contributors

• Lisa Lazareck
• Naomi Boxall
• Vladimir Prikazsky
• Aileen Kitching
• Sabrina Bacci