Are the benefits worth the risks?
Finally, for your patient, do the benefits of the therapy outweigh
any risks? One good expression of this is the Number Needed to Treat (NNT)
or the Number Needed to Harm (NNH). This is simply the recipricol of the
Absolute Risk Reduction (ARR).
In the case of a study that showed a 20% risk of death in the control group
and a 15% risk of death in the treatment group, the ARR = (20-25) = 5%.
The NNT = 100/5 = 20. This means you would have to treat 20 people to save the
life of one person.
However, if the risk of side effects is 10% then the NNH = 100/10 = 10. This means
that you would have to treat 10 people to cause 1 side effect. Therefore you
would be causing side effects in 2 people while saving the life of one person for
every 20 people you treated. This may sound pretty good but if applied
to other patients, treatments, outcomes, and side effects, with different risks, the
risks of therapy may outweigh the benefits.
Application to Pollock et al's
thyroxine paper
In this study, there was no measurable change in risk so NNT cannot be calculated.