# Statistical Guidelines and Checklist

### In the Methods section:

• Type and applicability of test used are stated

• Comparisons of interest are clearly defined

• Names of tests applied are clearly stated

• All statistical methods are identified unambiguously

• Justification for use of test is given

• Data meet all assumptions of tests applied (with particular attention to nonnormal

data sets or small sample sizes, which should be identified in the text as

such)

• Adjustments made for multiple testing are explained

### Details about the test

• N is reported at the start of the study and for each analysis thereafter

• Sample size calculation (or justification) is given

• Unit of analysis is given for all comparisons

• Alpha level is given for all statistical tests

• Tests are clearly identified as one- or two-tailed. If one-tailed, a justification is

necessary

• Actual P values are given for primary analyses

P values shall be reported to two decimal places

Note the following exceptions: Borderline P values (0.045 ≤ p < 0.055) can

be reported to three decimal places. P values < 0.01 can be reported to one

significant digit irrespective of the decimal places (<0.01, < 0.001, < 0.0001, etc.)

### Descriptive statistics summary

• N for each data set is clearly stated

• A clearly labeled measure of center (e.g., mean or median) is given

• A clearly labeled measure of variability (e.g., standard deviation or range) is given

• All numbers following a ± sign are identified as standard errors (s.e.m.) or

standard deviations (s.d.)

### Anomalies

• Any unusual or complex statistical method is clearly defined and explained for

the journal’s wide readership (authors are encouraged to use Supplementary

Information for long explanations)

• Any data exclusions are stated and explained

• Any discrepancies in the value of N between analyses are clearly explained and

justified

• Any method of treatment assignment (randomization, etc.) is explained and

justified

• Any data transformations are clearly described and justified

### Multivariable models

• The type of model should be clearly identified

• All variables included in the final model as adjusters shall be listed

• All variables that could have been included but were not due to a stepping

procedure should be listed, with a general description of why they were chosen

• If a stepping procedure is used, it should be explicitly described: e.g., forward,

backward, forward-and-backward stepwise, explicit hierarchical (forced

variables) or other

• Were assumptions of the model and goodness of fit assessed?

• Were interactions with the variable of primary study interest assessed?

• Main-effects estimates should not be reported if a substantial interaction is

detected unless the interaction is removed by a described method such as

stratification or centering

• When reporting parameter estimates (B, odds ratios, hazard ratios, etc.), 95%

confidence intervals as well as P values should be reported

### Tables and figures

• All tables and figures shall be referred to in the Results narrative and should be

numbered in the order in which they appear

• Data reported in tables and figures can be highlighted or summarized in the

Results narrative but should not be merely duplicated

• Tables and figures must be interpretable without reference to the text. Thus,

abbreviations need to be defined (perhaps in footnote or legend) even if

defined in text. N’s, including the denominators of reported %, should be clear

• P values that are reported in the body of the table or figure shall follow the

guidelines for P values above. However, for large tables of data not central to the

hypothesis, superscripted footnotes such as denoting <0.05 or <0.01 are allowed

### Interpretations

• Statistical significance should not be used as a synonym for truth. When

reporting findings that are significantly associated, keep in mind the possibility

of type I error. When reporting findings that are not significantly associated, take

into account the possibility of type II error. When “significant” or “not significant”

are used in the Results narative, the actual P value should be reported (even if in

parentheses). Usually, when reporting that variables are (or are not) associated

or correlated, the qualifier “significantly” should be included. Examples: “systolic

blood pressure and age were significantly associated (P = 0.03) ” or “diastolic

blood pressure and education were not significantly associated (P = 0.26)”.

### Within individual graphs:

**Distortions**

• Any distorted effect sizes (e.g., by truncation of y axis) are clearly labeled and

justified

**Clear labeling**

• Error bars are present on all graphs, where applicable

• All error bars are clearly labeled