JAMA, the JAMA Network journals and many other medical journals follow the AMA Manual of Style. Its statistics rules are short but differ from APA's in ways that trip up anyone switching between the two: a capital P, leading zeros on correlations, and a different confidence interval format. This guide collects the rules with examples.
P values
- Capital, italic P: P = .03.
- No leading zero (a probability cannot exceed 1): P = .03, not 0.03.
- Two decimal places when P ≥ .01: P = .03, P = .49.
- Three decimal places when P < .01: P = .004.
- The limits are P < .001 and P > .99; never P = .000 or P = 1.00.
- Report exact values, not P < .05, and always with the effect estimate.
Leading zeros
Put a zero before the decimal point for numbers below 1 (0.45, 0.58), except P, α and β. So, unlike APA, a correlation coefficient or a hazard ratio below 1 keeps its zero: r = 0.45, HR, 0.58.
Confidence intervals
- Comma after "95% CI" and a hyphen between the limits: (95% CI, 0.35-0.95).
- When either limit is negative, use "to" so the minus sign is not confused with the hyphen: (95% CI, −4.2 to −0.6).
- Give the CI with every main effect estimate: risk ratio, odds ratio, hazard ratio or mean difference.
Means, medians and percentages
- Mean (SD), not mean ± SD: "mean (SD) age, 62.7 (10.3) years".
- Median (IQR) with the quartiles as a range: "median (IQR) stay, 4 (2-7) days".
- No. (%): "54 (45.0%) were women"; give the denominator when it is not the whole sample.
- Use a consistent number of decimals, usually one for percentages and means.
Example sentences
The new treatment was associated with longer survival (hazard ratio, 0.58; 95% CI, 0.35-0.95; P = .03).
Mean (SD) systolic blood pressure fell by 8.4 (11.2) mm Hg in the intervention group and 2.1 (10.8) mm Hg in the control group (difference, −6.3 mm Hg; 95% CI, −10.2 to −2.4; P = .002).
AMA vs APA at a glance
| AMA | APA 7 | |
|---|---|---|
| P value | P = .03 (capital) | p = .03 (lowercase) |
| Leading zero on r | r = 0.45 | r = .45 |
| Confidence interval | (95% CI, 0.35-0.95) | 95% CI [0.35, 0.95] |
| Mean and SD | mean (SD), 62.7 (10.3) | M = 62.70, SD = 10.30 |
Mistakes reviewers catch
- Lowercase p in a JAMA-style manuscript.
- "±" for SD (or SE). Write mean (SD).
- APA-style brackets for confidence intervals.
- Three decimals for P values above .01, or two decimals for P = .004.
- P values without effect sizes and CIs.
Let KyroStat do the write-up for you
KyroStat writes results in the style your journal uses, with every P value, confidence interval and summary statistic formatted by its rules.
Frequently asked questions
Do BMJ and The Lancet use AMA style? No. Each has its own house style (The Lancet, for example, uses a raised decimal point). Check the journal's instructions for authors; the content you report is the same.
How do I format Table 1 in AMA style? See how to make Table 1.
How do I report a hazard ratio? See how to report a Cox regression.
What about APA journals? See how to report p values in APA 7.