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Commentary |

P Values vs Estimates of Association With Confidence Intervals

Peter Cummings, MD, MPH; Thomas D. Koepsell, MD, MPH
Arch Pediatr Adolesc Med. 2010;164(2):193-196. doi:10.1001/archpediatrics.2009.266.
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Since 1988, the International Committee of Medical Journal Editors has used this language in their guidelines for authors: “When possible, quantify findings and present them with appropriate indicators of measurement error or uncertainty (such as confidence intervals). Avoid relying solely on statistical hypothesis testing, such as the use of P values, which fails to convey important quantitative information.”1,2 Hundreds of biomedical journals, including the Archives,3 endorse these guidelines. What concerns do editors have about P values and hypothesis testing?

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Figure 1.

Plot of 2-sided P values for a set of risk ratios based on data from the trial of drug D (Table 3). Each P value is for a hypothesis test that each risk ratio from 0.125 to 2.0 is true in the population from which the study subjects came. The vertical line marks the null risk ratio of 1.0 and the dashed horizontal line marks both the 95% confidence level and the P value of .05. The 95% confidence limits are risk ratios 0.22 and 1.13.

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Figure 2.

Plots of 2-sided P values for risk ratios from 2 studies. The solid P value curve is based on data from a small study of catheters impregnated with drug A (Table 1) and the dashed curve is based on data from a large study of drug A catheters (Table 4). The vertical line marks the null risk ratio of 1.0 and the dashed horizontal line marks both the 95% confidence level and the P value of .05.

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