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.