Relative risk from a 2×2 table
Treatment is the top row. A relative risk under 1 means the treatment row had the lower event rate.
For the sign of NNT, use the number needed to treat calculator.
Questions
How is relative risk calculated?
Relative risk is the event rate in the treated group divided by the event rate in the control group. A value under 1 means fewer events on treatment.
What is the difference between relative risk and odds ratio?
Relative risk uses event rates. The odds ratio uses the odds of the event. They are close when events are rare and they pull apart when events are common.
Where is the number needed to treat?
Number needed to treat is 1 divided by the absolute risk difference. This page shows it, and the dedicated number-needed-to-treat calculator explains the sign.
Educational only. Not medical advice, not a regulatory determination, and not a substitute for qualified RA/QA, legal, clinical, or statistical counsel. Relative risk calculator does not submit to FDA, Notified Bodies, or IRBs.