Table 1:

Relevant reporting guidelines related to testing group imbalances for confounder selection in nonrandomized studies

Organization/external reporting guideline	Current guidance
ICMJE9	Does not include guidance on reporting of research methods including testing group imbalances for confounder selection in nonrandomized studies. Refers authors to STROBE.
STROBE10,11	Item 12 (a). Describe all statistical methods, including those used to control for confounding “Investigators should think beforehand about potential confounding factors. This will inform the study design and allow proper data collection by identifying the confounders for which detailed information should be sought.” “If groups being compared are not similar with regard to some characteristics, adjustment should be made for possible confounding variables by stratification or by multivariable regression.”
	Item 14: Descriptive data “Inferential measures such as standard errors and confidence intervals should not be used to describe the variability of characteristics, and significance tests should be avoided in descriptive tables. Also, P values are not an appropriate criterion for selecting which confounders to adjust for in analysis; even small differences in a confounder that has a strong effect on the outcome can be important.”
TREND12	Item 15: Baseline equivalence — data on study group equivalence at baseline and statistical methods used to control for baseline differences “Example (baseline equivalence): the intervention and comparison groups did not statistically differ with respect to demographic data (gender, age, race/ethnicity; p > .05 for each), but the intervention group reported a significantly greater baseline frequency of injection drug use (p = .03); all regression analyses included baseline frequency of injection drug use as a covariate in the model.”
GRADE13	5.2 Factors that can reduce the quality of the evidence 5.2.1 Study limitations (risk of bias) “Study limitations in observational studies”: Failure to develop and apply appropriate eligibility criteria (inclusion of control population) ○ Under- or overmatching in case–control studies ○ Selection of exposed and unexposed in cohort studies from different population Failure to adequately control confounding ○ Failure of accurate measurement of all known prognostic factors ○ Failure to match for prognostic factors and/or adjustment in statistical analysis 5.3. Factors that can increase the quality of the evidence 5.3.3. Effect of plausible residual confounding “Rigorous observational studies will accurately measure prognostic factors associated with the outcome of interest and will conduct an adjusted analysis that accounts for differences in the distribution of these factors between intervention and control groups.”
SAMPL14	Does not include specific guidance on reporting of research methods including testing group imbalances for confounder selection in nonrandomized studies. Refers authors to STROBE and TREND.

Note: GRADE = Grading of Recommendations Assessment, Development and Evaluation, ICMJE = International Committee of Medical Journal Editors, SAMPL = Statistical Analyses and Methods in the Published Literature, STROBE = STrengthening the Reporting of OBservational studies in Epidemiology, TREND = Transparent Reporting of Evaluations with Nonrandomized Designs.