Instructor Resource

Chapter 17

Copyright © Scott B. Patten, 2015.

Permission granted for classroom use with Epidemiology for Canadian Students: Principles, Methods & Critical Appraisal (Edmonton: Brush Education Inc. www.brusheducation.ca).

Chapter 17. Other measures of association in epidemiology

Objectives

• Define following measures of association: • attributable risk among the exposed• attributable risk percent• population attributable risk • population attributable risk percent (attributable

fraction)

• Describe how to interpret these measures of association.

Attributable risk

• This chapter covers other measures of association that convey important and useful epidemiological information. • Most of these measures are estimates based on risk

differences, but they are intended to quantify more than just an association. They are intended to quantify the causal impact of a risk factor on a population .

Attributable risk among the exposed• This quantifies the extent to which risk in

exposed subjects could be diminished if the effects of the exposure were eliminated.

where AR = attributable risk and IP = incidence proportion

Attributable risk and causality• Labelling a risk difference an attributable risk gives

it a causal interpretation. • There are situations where this is not justified. For

example, if the difference between the 2 incidence proportions could arise from a confounding variable, the risk may not be attributable to the exposure. • Caution is required in the use of this terminology.

Attributable risk percent

• Some epidemiologists prefer to present estimates of attributable risk among the exposed as percentages rather than simple risk differences.

where AR = attributable risk and IP = incidence proportion

An alternative formula

• This formula allows AR% to be approximated using an approximation of the risk ratio, such as an odds ratio from a case control study.

where AR = attributable risk

Additional terminology

• Some epidemiologists refer to the attributable risk percent as the attributable fraction, and it is not necessarily expressed as a percentage. • Another similar term is attributable proportion. • Others specify that this as the attributable fraction for

the exposed to clarify that it reflects impact on those who are exposed and not the entire population.• The key point is that these parameters help to predict

the potential impact of primary prevention on the health status of an exposed population.

Preventive fraction

• When an exposure diminishes risk, it is more convenient to express its effects as a preventive fraction.

where PF = preventive fraction and IP = incidence proportion

Attributable and preventive fractions• Attributable risk answers the question: “How much

disease could be prevented by removing a harmful factor among those exposed to it?” • Preventive fraction asks: “How much disease

among the nonexposed could be prevented by adding the exposure to the nonexposed?”

An alternative formula

• This is another way to calculate the preventive fraction (PF).

Population attributable risk

• This helps assess the effect of primary-prevention interventions on an entire population.

where PAR = population attributable risk and IP = incidence proportion

Population attributable risk (continued)• Here, the effect of exposure in the population is

represented as the difference between incidence in the total population (which in turn reflects the effect of the exposure in question and all other factors) and incidence in the nonexposed (which reflects the risk of all other factors in the absence of the exposure). • If the exposure could be eliminated, the incidence

in the total population would be that of the nonexposed.

Population attributable risk percent

• Population attributable risk can also be expressed as a fraction or a percentage.

where PAR = population attributable risk and IP = incidence proportion

Summary

• Population attributable risks are a family of parameters that combine risk differences with judgements about causation to predict the potential impact of primary prevention on the health status of the total population.

End