Numbers needed to treat (NNT) is a measure that indicates how many patients would require an intervention to reduce the expected number of outcomes by one.

It is calculated by 1/(Absolute risk reduction) and is rounded to the next highest whole number

Experimental event rate (EER) = (Number who had particular outcome with the intervention) / (Total number who had the intervention)

Control event rate (CER) = (Number who had a particular outcome with the control/ (Total number who had the control)

Absolute risk reduction = CER-EER or EER-CER?

The absolute risk reduction (ARR) may be calculated by finding the absolute difference between the control event rate (CER) and the experimental event rate (EER). You will often find both versions of the above listed in different sources. In some ways in doesn't matter which you use as you will end up with the same answer but from a technical point of view:

• if the outcome of the study is undesirable then ARR = CER - EER
• if the outcome of the study is desirable then ARR* = EER - CER
• this may be more accurately termed absolute benefit increase, rather than absolute risk reduction

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A 64-year-old woman, whose husband had a TIA one month ago brings you a newspaper article with the headline 'new super drug prevents stroke'. Reading through the article with her, it states that a recent clinical trial has shown that a new lipid-lowering therapy for stroke had a number needed to treat (NNT) of 20 for the prevention of the primary end-point. How do you best describe these results to her?

1. 20 extra patients in the placebo group had a stroke
2. For 1000 patients treated with active therapy, there would be 50 fewer strokes
3. For 1000 patients treated with active therapy, there would be 20 fewer strokes
4. For every 1000 patients treated with active therapy there would be 100 fewer strokes
5. 20 patients in the treatment group were protected from stroke

• Correct Answer

Explanation: