How to report a P-value

An example

If you've suffered from motion sickness, you might have taken a medication (such as "Kwells") based on hyoscine hydrobromide.

In the early 20th century, the effect of hyoscine hydrobromide on sleep was investigated. In his original paper deriving the t-distribution, Student illustrated his methods using data from a study looking at the gain in hours of sleep after using various forms of hyoscine hydrobromide.  Here is a dotplot of the hours gained when the patients used a particular form - Laevo-hyoscine hydrobromide.  The measurement is the hours gained - the difference between using Laevo-hyoscine hydrobromide and not using it.

Here we consider carrying out a single-sample t-test of the null hypothesis that the true mean hours gained was zero.

How to report P-values

• If you are reporting P-values in an academic paper or thesis, it's good practice to report the actual value to three decimal places.
• If the P-value is very small, common practice is to report it as P < 0.001.
• It's not sufficient to only report a P-value; relevant estimates and confidence intervals should also be provided.

For example, patients gained an average of 2.3 hours of sleep when using Laevo-hyoscine hydrobromide, with a 95% confidence interval of 0.9 to 3.8 hours. The P-value for a test of the null hypothesis that the true average gain was zero hours was P = 0.005.

You will find the statistical results in the software output below.