Guidelines for reporting non-randomised studies.

Non-randomised studies (NRSs) are useful because they allow interventions to be evaluated that are difficult to investigate by randomised controlled trials (RCTs). However, NRSs are more susceptible to bias. The Consolidated Standards of Reporting Trials (CONSORT) statement was established to ensure that researchers report features of RCTs that must be considered when appraising their quality. CONSORT has improved the reporting of key information, highlighting missing key information for users. Researchers have a responsibility to report essential information that allows users to assess the susceptibility of NRS to selection, performance, detection and attrition bias. This paper considers criteria for reporting cohort studies: the rationale behind the CONSORT criteria for reporting of RCTs will be applied to cohort studies. Many of the criteria need no modification but application of others raise difficult issues for cohort studies, e.g.: description and standardisation of control and intervention treatments; description of the method of allocation; choice of prognostic factors to be collected; distinguishing between intended and provided treatments; collection of data on adverse and longterm outcomes; establishing a priori plans for analysis.