This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  Computing confidence limits

Confidence limits for ARRs and NNTs may be calculated by applying the above formulae to the upper and lower confidence limits for the summary statistic (RD, RR or OR) (Altman 1998). Note that this confidence interval does not incorporate uncertainty around the control group risk (CGR).


In the case of what conventionally are considered non-statistically significant results (for example, the 95% confidence interval of OR or RR includes the value 1) one of the confidence limits will indicate benefit and the other harm. Thus, appropriate use of the words ‘fewer’ and ‘more’ is required for each limit when presenting results in terms of events. For NNTs, the two confidence limits should be labelled as NNTB and NNTH to indicate the direction of effect in each case. The confidence interval for the NNT will include a ‘discontinuity’: within the interval there will be an infinitely large NNTB, which will switch to an infinitely large NNTH.