Table 1

Rounding rules for summary statistics

Summary statistic	Reporting	Examples (where useful)
Mean	Use enough decimal places to give either the SD to two significant digits,7 or the SE to one significant digit	3320 g
Mean		3.32 kg
Percentage	Integers, or one decimal place for values under 10%. Values over 90% may need one decimal place if their complement is informative. Use two or more decimal places only if the range of values is less than 0.1%	0.1%
		5.3%
		27%
		89%
		99.6%
Mean difference	Use enough decimal places to give the SE to one or two significant digits. For a standardised mean difference use one or two decimal places
Regression coefficient	As with the mean difference.
Correlation coefficient	One or two decimal places, or more when very close to ±1	0.03
		−0.7
		0.89
		0.999
Risk ratio	Round to two significant digits if the leading non-zero digit is four or more, otherwise round to three (the rule of four11). Alternatively use one/two significant digits rather than two/three. For ORs very close to 1 (eg, in logistic regression with a continuous variable) use three decimal places or else report the log OR×100 as the percentage odds to one decimal place13	0.0321
		0.062
		0.76
		1.05
		4.2
		11.3
		55
		1.042
		4.1%
SD	One or two significant digits7	570 g
		0.57 kg
		9 mm Hg
		2.5 mL
SE	One or two significant digits
CI	Use the same rule as for the corresponding effect size (be it mean, percentage, mean difference, regression coefficient, correlation coefficient or risk ratio), perhaps with one less significant digit
Test statistics: t, F, χ², etc	Up to one decimal place and up to two significant digits
		t=−1.3
		F=11
		χ^s=4.1
p value	Round up to one significant digit, within the limits shown in the examples. The lower limit may be smaller than 0.001, but never 0.000. For genome-wide association studies use the power of 10 format	>0.9
		0.4
		0.1
		0.08
		0.05
		0.003
		<0.001
		6.10⁻⁹