BMJ · 2011 · 484 citations · 2 references
Natural SciencesBiostatisticsMeta-analysisStatisticsUncertainty QuantificationConfidence IntervalsEffect Size EstimationHypothesis TestingConfidence Interval AnalysisConfidence Interval ConstructionConfidence Interval EstimationConfidence Interval EvaluationConfidence Interval InterpretationConfidence Interval MethodologyConfidence Interval PrecisionP-values
Some studies report confidence intervals without accompanying P values. This paper demonstrates how to derive a P value from a confidence interval when needed. The authors outline a two‑case procedure for computing a P value from a confidence interval, detailing steps for effect estimates. The authors confirm that a confidence interval can be converted to a P value, note its usefulness when P values are imprecise, and advocate for confidence intervals over P values.
We have shown in a previous Statistics Note 1 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. #### Steps to obtain the P value from the CI for an estimate of effect ( Est ) ##### (a) P from CI for a difference
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