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Chapter 1 - Risk Factors

Risk is defined as the number of events divided by the number of people at risk. When measured over a specified period of time, it is described as the incidence rate. Differences in risk due to an exposure may be expressed as a ratio or a difference.


Number of events/number of people at risk

Risk ratio

Risk of exposed/risk of unexposed

• measures the strength of the effect

• is independent of the population risk

Risk difference

Risk of exposed - risk of unexposed

• describes the number of additional cases due to the exposure

Excess or attributable risk (Parkin, 2011; Whiteman et al, 2015)

• is the difference in the risk of a condition between an exposed population and an unexposed population

Risk ratio and risk difference

In a study of hormone replacement therapy (HRT) (Jones et al, 2016), 500 out of 20 114 non-users and 52 out of 1612 users of combined HRT developed breast cancer (Table 1). The risk to users was 3.6% and to non-users 2.5%, giving a risk ratio of 1.30 (i.e. the risk to users was 30% greater). The difference in risk was 0.74%, equivalent to 12 (1612 × 0.74%) additional cases of cancer in the 1612 users.

Table 1: Relative Risk of Postmenopausal Breast Cancer, by Type of HRT Preparation

All women


Risk (%)


20 114


500/20 114=2.5%







Risk ratio (3.2%/2.5%)


Risk difference (3.2%–2.5%)


From Jones ME, Schoemaker MJ, Wright L, et al. Menopausal hormone therapy and breast cancer: what is the true size of the increased risk? Br J Cancer 2016; 115:607-615.


Attributable risk: Proportion of the disease or outcomes in a group who have been exposed to a particular factor, which can be attributed to exposure to that factor.

Incidence rate: Number of new cases of a particular disease diagnosed during a certain period of time.

Risk: The proportion of patients in a group who have the event of interest.

Risk difference: The difference in the risk that an event occurred in one group (usually the intervention or exposure group) and the risk of the event in a second group (usually the control group).

Risk ratio: The ratio of the risk that an event occurred in one group (usually the intervention or exposure group) to the risk of the event in a second group (usually the control group).

Chapter 1 - Risk Factors Causation

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