How to obtain the P value from a confidence interval.

We have shown in a previous Statistics Note how we can calculate a confidence interval (CI) from a P value. Some published articles report confidence intervals, but do not give corresponding P values. Here we show how a confidence interval can be used to calculate a P value, should this be required. This might also be useful when the P value is given only imprecisely (eg, as P<0.05). Wherever they can be calculated, we are advocates of confidence intervals as much more useful than P values, but we like to be helpful. The method is outlined in the box below in which we have distinguished two cases. (a) P from CI for a difference (no transformation needed)