Question 25

In the context of clinical trials, define the following terms:

(a)      Relative risk
(b)      Absolute risk
(c)       Number needed to treat
(d)       Power of the study

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College Answer

A number of potential definitions exist. One example for each is listed below:
Relative risk: the difference in event rates between 2 groups expressed as proportion of the event rate in the untreated group.

Absolute risk: this is the actual event rate in the treatment or the placebo group. The absolute risk reduction is the arithmetical difference between the event rates between the two groups

Number Needed to Treat: The NNT is the number of patients to whom a clinician would need to administer a particular treatment for 1 patient to receive benefit from it. It is calculated as 100 divided by the absolute risk reduction when expressed as a percentage or 1 divided by the absolute risk reduction when expressed as a proportion.
Power of the study: The probability that a study will produce a significant difference at a given significance level is called the power of the study. It will depend on the difference between the populations compared, the sample size and the significance level chosen.


Thes question is a verbatim copy of Question 9 from the second paper of 2010.