Number needed to treat (NNT)
The inverse of the absolute risk reduction or increase and the number of patients that need to be treated for one to benefit compared with a control. The ideal NNT is 1, where everyone has improved with treatment and no-one has with control. The higher the NNT, the less effective is the treatment. But the value of an NNT is not just numeric. For instance, NNTs of 2-5 are indicative of effective therapies, like analgesics for acute pain. NNts of about 1 might be seen by treating sensitive bacterial infections with antibiotics, while an NNT of 40 or more might be useful, as when using aspirin after a heart attack.
An example might help. NNT should always be given with the control treatment, the intervention used, the intensity (dose) and duration of the intervention, the outcome, and the period over which observations were made for that outcome.