Portal de Periódicos da CAPES

How to obtain the confidence interval from a P value

2011; Volume: 343; Issue: aug08 1 Linguagem: Inglês

10.1136/bmj.d2090

1756-1833

Douglas G. Altman, Martin Bland,

Healthcare cost, quality, practices

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