The words to use
- Absolute risk reduction (ARR)
- The risk without treatment minus the risk with it. A fall from 4% to 2% is an ARR of 2 percentage points.
- Number needed to treat (NNT)
- How many people need the treatment, over a stated time, for one to benefit: 100 ÷ ARR in percentage points.
- Relative risk
- The risk in one group divided by the risk in another. A relative risk of 2 means twice as likely, but says nothing about how common the event is.
- Confounding
- A third factor, linked to both the exposure and the outcome, that creates or hides an apparent effect. Randomisation is the best protection against it.
- Case series
- A report of several patients with a condition and no comparison group. Useful for raising questions, not for proving cause.
