1. 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).

2. Chapter 17. Other measures of association in epidemiology

3. 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.

4. 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.

5. 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

6. 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.

7. 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

8. 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

9. 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.

10. Preventive fraction where PF = preventive fraction and IP = incidence proportion

11. 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?”

12. An alternative formula This is another way to calculate the preventive fraction (PF).

13. 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

14. 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.

15. 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

16. 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.

17. End