Calculating risk from clinical trial outcomes

The data used in the following examples are adapted with the permission of Australian Prescriber (Aust Prescr 2008;31:12–6).¹¹

Example 1

In a trial of 441 patients at risk of developing pressure ulcers, patients were randomised to receive a sheepskin mattress overlay (the intervention group) or usual treatment (the control group) during their hospital stay. Patients developing pressure ulcers: trial data shows the results after 20 days of observation.

The absolute risk reduction can be calculated by subtracting the proportion of patients with ulcers in the sheepskin group from that in the control group:

The absolute risk of developing ulcers in the sheepskin group was thus 7% less than that in the control group.

Relative risk can also be calculated from the data in the table:

In the trial, 10% of patients in the sheepskin group developed ulcers, compared with 17% in the control group. So the risk of developing ulcers in the group with a sheepskin overlay was 0.58 of that in the control group.

The relative risk reduction in the trial can be calculated as follows:

Sheepskin overlays reduced the relative risk of patients getting ulcers by 42%.

The odds ratio can be calculated by dividing the odds of developing an ulcer in the sheepskin group by the odds in the control group:

Therefore, the odds of developing an ulcer in the sheepskin group were 0.54 of that in the control group—that is, patients with a sheepskin overlay were half as likely to develop ulcers as patients given the usual treatment.

The number needed to treat can be calculated as the reciprocal of absolute risk reduction:

Fourteen patients need to have a sheepskin overlay for 20 days instead of the usual treatment to prevent one of them from developing an ulcer.

Example 2

In an overview of RCTs of hypertension management, rates of stroke were measured in patients randomised to receive the experimental treatment or control. Results were analysed according to the severity of hypertension (Relationship between relative risk, absolute risk and odds ratio).