P-Value Pointers

How to format P values in AMA style

In this post, I’ll be going over P values and how to express them according to the AMA Manual of Style.

Many of you probably already know this, but I’ll first briefly define what a P value is. When you are comparing a treatment with a placebo, for example, you will have a null hypothesis, which states that the treatment is no better than placebo. This is in contrast to the alternative hypothesis, which states that the treatment is better than placebo.

A low P value (generally defined as P < .05) suggests that your sample provides enough evidence that you can reject the null hypothesis for the entire population. It does not measure support for the alternative hypothesis, but it is saying that your result, if your P value is < 0.05, would be very unlikely if the treatment were, in reality, no better than placebo. You could say that if P < 0.05, that the probability that you would get your result by chance alone is < 5%.

This is not by any means a complete explanation of P values. There’s a lot to it, and of course if you’re interested in learning more, then please check out your favorite statistics textbook.

Now, when it comes to expressing and styling P values, we have to do that all day long as medical writers. Here’s a couple of pointers that will helpyou align the formatting according to the AMA Manual of Style. We’ll mainly be referring to sections 20.8.2 and 20.9.

Express P Values to 2 Digits After the Decimal

So first of all, P values should be expressed to 2 digits to the right of the decimal point (regardless of whether the P value is significant), unless P < .01, in which case the P value should be expressed to 3 digits to the right of the decimal.  So, for example, for P < .0085 you would write as P < .0085 not P < .01. Also when rounding P from 3 digits to 2 digits would result in P appearing nonsignificant, such as P  = .046, expressing the P value to 3 places may be preferred.

Smallest P Value Is P < .001

The smallest P value that should be expressed is P < .001 since additional zeros do not convey useful information. So, if you had a P value of .0001, you would still express it as P < .001.

Certain Study Types Do Require Expressing P to More Than 3 Digits.

An update was made in 2011: “Although our style manual recommends (Section 20.9) that “[expressing] P to more than 3 significant digits does not add useful information to P < .001,” in certain types of studies (particularly GWAS [genome-wide association studies] and other studies in which there are adjustments for multiple comparisons, such as Bonferroni correction, and the definition of level of significance is substantially less than P < .05) it may be important to express P values to more significant digits. For example, if the threshold of significance is P<.0004, then by definition the P value must be expressed to at least 4 digits to indicate whether a result is statistically significant. GWAS express P values to very small numbers, using scientific notation. If a manuscript you are editing defines statistical significance as a P value substantially less than .05, possibly even using scientific notation to express P values to very small numbers, it is best to retain the values as the author presents them.

Watch Out for Trends

Also, when you have a P value that just misses statistical significance, for example, P = 0.06, watch out for language from yourself or others describing the findings as “trending toward significance,” “having a trend toward significance,” “approaching significance,” “borderline significant,” or “nearly significant.” None of these terms is correct. Results do not trend toward significant—they either are or are not statistically significant based on the prespecified study assumptions. The term trend should only be used when reporting the results of statistical tests for trend. Other uses of trend or approaching significance should be removed and replaced with a simple statement of the findings and the phrase not statistically significant.

P values Should Not Be Listed as Not Significant

You should include a valuebecause for a meta-analysis the actual values are important and not providing exact P values is, according to the AMA Manual of Style, a form of incomplete reporting.

Format Correctly

Finally, you want to format the P value correctly. The zero before the decimal point is omitted. There is a space either side of the < or = sign and the P is italicized.

Well that’s it for now. If you need medical writing help, please check out our website nascentmc.com, and if you are interested in getting into medical writing, please visit 6weekcourse.com.