Cochrane Style Manual
Statistical and mathematical presentation

This section is being reviewed and is likely to be revised during 2017.

This section provides general guidance on the presentation of statistical and mathematical terms and values. Please also refer to the Cochrane Handbook for Systematic Reviews of Interventions.

For guidance on abbreviating statistical terms commonly used in Cochrane Reviews see Common abbreviations.

 

Guidance

Correct

Incorrect

Decimal places

Odds ratios, risk ratios, and standardized mean differences should usually be quoted to two decimal places.

For very large or very small values, use judgement to determine whether fewer or more decimal places should be used to express the appropriate level of precision.

12.26

12.3

Decimal points

Use full stops, not commas.

15.51

15,51

Equals sign

For values that are represented by a mathematical symbol (e.g. P and I2) use an equal sign and do not include the word "value" when used to present a value. For terms that are represented as abbreviations (e.g. RR, OR, MD) do not use an equals sign.

P = 0.015

I2 = 20%

RR 0.05

P value = 0.015
P 0.015

I2 20%

RR value 0.05
RR = 0.05

Mathematical equations

 

Avoid building equations or formulae spaced over two or more lines in the text of the review, as text formatting will change during publication process.

2 = 10/5

      10
2 = ---
       5

Use spaces either sides of '-', '+', and '=' symbols in mathematical equations.

See also: Guidance on spacing around commonly used symbols

SE = sqrt((1/r1) + (1/(n1 − r1)) + (1/r2) + (1/(n2 − r2)))

SE = sqrt((1/r1)+(1/(n1−r1)) +(1/r2)+(1/(n2−r2)))

Number needed to treat (NNT)

Express all NNTs as positive whole numbers, all decimals being rounded up.

Use 'number needed to treat for an additional beneficial outcome' and its abbreviation 'NNTB', not 'number needed to treat' or 'NNT'. Similarly, use 'number needed to treat for an additional harmful outcome’ (NNTH) to specify a harmful event.

NNTH

NNT 10

NNH

NNT 10.5

P value

P values should be stated exactly, apart from values less than 0.001, which should be expressed as P < 0.001.

P = 0.03

P < 0.001

P < 0.05

P = 0.0005

Use the phrase 'P value' in text if referring to the statistic, but use 'P = ' when presenting a value.

We calculated the P value.

P = 0.05

We calculated P.

P value = 0.05

Use an upper-case ‘P’ (not italic), and do not add hyphen between the ‘P’ and the value

P value

p value

P value

P-value

p-value

P values should be expressed with two significant figures and up to three decimal places.

P = 0.23

P = 0.051

P = 0.003

P = 0.001

P = 0.232

P = 0.05

P = 0.0025

P = 1.3 x 10-3

Sample and population sizes

For dichotomous outcomes, use the headings n/N within each intervention arm, where n denotes the number of events and N is the sample size. It is preferable to standardize the use of n/N for these where possible.

Summary statistic and confidence interval

 

 

 

Only use abbreviations for summary statistics (e.g. RR or MD) and confidence interval (CI) if they have already been defined (see Abbreviations and acronyms for further guidance).

The risk ratio (RR) was 0.38 (95% confidence interval (CI) 0.30 to 0.49)

Separate summary statistic from its CI using a comma if inside a single set of brackets.

…was statistically significant (RR 0.09, 95% CI 0.02 to 0.38)

Define the CI (e.g. 95% or 99%).

(odds ratio 1.11, 95% CI 0.98 to 1.20)

(odds ratio 1.11, CI 0.98 to 1.20)

Separate the CIs with ‘to’ instead of using a hyphen.

(mean difference −11.11 hours, 95% CI −20.04 to −2.18; number of participants 23)

(mean difference −11.11 hours; 95% CI −20.04- −2.18; number of participants 23)