A practical guide for understanding confidence intervals andPvalues

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Abstract

The 95 percent confidence interval about the mean demarcates the range of values in which the mean would fall if many samples from the universal parent population were taken. In other words, if the same observation, experiment, or trial were done over and over with a different sample of subjects, but with the same characteristics as the original sample, 95 percent of the means from those repeated measures would fall within this range. This gives a measure of how confident we are in the original mean. It tells us not only whether the results are statistically significant because the CI falls totally on one side or the other of the no difference marker (0 if continuous variables; 1 if proportions), but also the actual values so that we might determine if the data seem clinically important. In contrast, the P value tells us only whether the results are statistically significant, without translating that information into values relative to the variable that was measured. Consequently, the CI is a better choice to describe the results of observations, experiments, or trials.

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