How to obtain the confidence interval from a P value

Confidence intervals (CIs) are widely used in reporting statistical analyses of research data, and are usually considered to be more informative than P values from significance tests.1 2 Some published articles, however, report estimated effects and P values, but do not give CIs (a practice BMJ now strongly discourages). Here we show how to obtain the confidence interval when only the observed effect and the P value were reported. The method is outlined in the box below in which we have distinguished two cases. #### Steps to obtain the confidence interval (CI) for an estimate of effect from the P value and the estimate ( Est ) ##### (a) CI for a difference ##### (b) CI for a ratio