How to obtain the P value from a confidence interval

• Published in: BMJ Vol. 343; no. aug08 1; p. d2304
• Main Authors: Altman, Douglas G, Bland, J Martin
• Format: Journal Article
• Language: English
• Published: England British Medical Journal Publishing Group 08.08.2011; BMJ Publishing Group LTD
• Subjects: Confidence Intervals; Probability
• ISSN: 0959-8138; 1756-1833; 1468-5833; 1756-1833
• DOI: 10.1136/bmj.d2304

Steps to obtain the P value from the CI for an estimate of effect (Est) (a) P from CI for a difference If the upper and lower limits of a 95% CI are u and l respectively: 1 calculate the standard error: SE = (u - l)/(2×1.96) 2 calculate the test statistic: z = Est/SE 3 calculate the P value 2 : P = exp(-0.717×z - 0.416×z2). (b) P from CI for a ratio For a ratio measure, such as a risk ratio, the above formulas should be used with the estimate Est and the confidence limits on the log scale (eg, the log risk ratio and its CI). In this paper, the authors did indeed publish a P value of 0.13, 4 as we have estimated from their confidence interval. (b) CI for a ratio (log transformation needed) The calculation is trickier for ratio measures, such as risk ratio, odds ratio, and hazard ratio. All authors have completed the Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available on request from the corresponding author) and declare: no support from any organisation for the submitted work; no financial relationships with any organisations that might have an interest in the submitted work in the previous 3 years; no other relationships or activities that could appear to have influenced the submitted work. 1 Altman DG, Bland JM.