Don't Overemphasize P Values
When you write up your results, a few key pointers:
- Consider emphasizing the effect size and confidence interval, rather than a P value. The effect size can be a difference, a ratio, or a correlation coefficient, all of which can be reported with a 95% confidence interval. This is often more useful than P values alone.
- Don't just say whether the P value is greater or less than 0.05. If you can, give the P value as a number.
- With multiple comparisons, it is not possible to give individual P values for each comparison, but it is possible in many cases to report multiplicity-adjusted P values.
- If you computed many P values, show them all. At a minimum, state the number of comparisons you made. Reporting only comparisons with small P values creates misleading results.
- It doesn't help to report a P value unless you clearly state which test was used to compute it.
Asterisks
Many people add asterisks to tables and graphs to show how low the P value is. The standards for one to three asterisks are widely accepted (<0.05, <0.01, <0.001). Prism (since version 5.04/d) also shows four asterisks when the P value is less than 0.0001.
| P value | APA | NEJM | Prism up to 5.04/d | Prism 5.04/d and later |
|---|---|---|---|---|
| 0.04 | * | * | * | * |
| 0.009 | ** | ** | ** | ** |
| 0.0009 | *** | *** | *** | *** |
| 0.00009 | *** | *** | *** | **** |
Make Sure the Null Hypothesis Is Clear
Every P value tests a null hypothesis, so your readers need to know what the P value is testing. If you place an asterisk on a graph, it must be clear exactly what comparison was made. Without that context, the asterisk is meaningless.
How Many Digits? Leading Zero or Not?
Three reporting styles are in common use: APA, NEJM, and GraphPad. They differ in whether a leading zero is included and how many decimal places are shown.
| P value | APA | NEJM | GraphPad |
|---|---|---|---|
| 0.1234 | ".123" | "0.12" | "0.1234" |
| 0.01234 | "0.012" | "0.012" | "0.0123" |
| 0.00123 | ".001" | "0.001" | "0.0012" |
| 0.00012 | "<.001" | "<.001" | "0.0001" |
| 0.00001 | "<.001" | "<.001" | "<0.0001" |
APA: Omits the leading zero before the decimal point (e.g. ".123" not "0.123"). Reports two or three digits after the decimal. Rationale: values that by definition cannot exceed 1.0 omit the leading zero as a visual indicator.
NEJM: P values larger than 0.01 reported to two decimal places; between 0.01 and 0.001 to three decimal places; smaller than 0.001 reported as P < 0.001.
GraphPad: Always reports a leading zero and four digits after the decimal. Values less than 0.0001 are shown as "<0.0001".
"P value" or "p value"?
There is no universally agreed style:
- APA: "p value" — lowercase, italicized p, no hyphen.
- NEJM and GraphPad: "P value" — uppercase P, not italicized, no hyphen.
- Some journals: "p-value" — note the hyphen. Check your target journal's style guide.