Randomized controlled trials (RCTs) with multiple arms are sometimes considered an attractive way of optimizing resources and simultaneously testing various treatment strategies [1–4]. For instance, multiple-arm trials can involve increasing doses of an experimental treatment, cumulative combination therapies, or multiple independent treatments, which can allows testing of the efficacy of new treatments to be carried out more rapidly and more directly [5]. Such trials provide more information than two-arm trials can provide [6]. Multiple-arm randomized trials are becoming increasingly common, with a quarter of randomized trials having more than two intervention groups [7].

However, because of the number of arms, such trials can be more complex in design, data analysis and result reporting compared with two-arm trials [2, 8, 9]. Complications of such trials are directly related to the number of arms and the number of possible comparisons. For instance, in an RCT with three arms, there are seven theoretically possible comparisons [6]. The complications of such trials include: defining a priori which comparisons are of primary interest; the possibility of performing global comparison tests (that is, assessing global differences between all arms) and/or pairwise comparison tests (that is, assessing differences of 2 arms), of pooling data for two or more arms, of reporting selective comparisons (for example, only statistically significant comparisons) or post hoc comparisons (for example, comparisons that were not planned in the protocol), or of using a multiple comparison adjustment procedure for controlling type I error rate, which influences sample-size calculation and statistical analysis; and the necessity of having sufficient details of the primary outcomes per group for future meta-analyses. To our knowledge, no systematic review has compared the planned comparisons (as described in reports) and the reported comparisons from multiple-arm trials. We aimed to appraise the reporting of analyses from RCTs with multiple arms by examining a sample of reports of results of such trials published in core clinical journals.

Our findings highlight the inadequate reporting of baseline characteristics and outcomes for arms in multiple-arm RCTs, and the discrepancies between planned and reported comparisons. Moreover, such trials generally had relatively small sample sizes, and showed great variability in the types of intervention and the control arms used.

Multiple-treatment arms are possible sources of multiplicity in an RCT [6, 10]. This multiplicity is related to the possibility of performing several pairwise tests to determine the most effective arm. In such a setting, the objectives of the trial must be clear to ensure that these objectives (and only these) are correctly designed and analyzed (for example, global test comparison or not, and which pairwise test comparisons are planned). In 20% of the reports we analyzed, the study hypothesis was not described, which suggests selective reporting. Moreover, bias may be introduced if the decisions on data analysis are driven by the data [11]. For instance, groups receiving different doses of the same intervention could be combined after the data are examined, or only statistically significant pairwise comparisons could be reported. The number of possible comparisons increases greatly in trials with more than three arms, which suggests increased risk of selective reporting. Our results are likely have underestimated any selective outcome reporting bias because we assessed articles and not protocols [12].

Moreover, in our study, some reports did not describe baseline or outcome data for each group (occurred in more than 15% of reports for each scenario). These results are consistent with previous work [13], and are important because reporting data per group is a necessary condition for future meta-analyses.

One of the other methodological difficulties in multiple-arm RCTs concerns the calculation of the sample size, and particularly the necessity for adequate power. Many randomized trials with two parallel arms exhibit inadequate power for revealing differences [14], and sample-size calculation is poorly reported in articles of trials and can be inaccurate [15]. With multiple-arm trials, problems with power and calculation are enhanced, particularly because sample-size calculation depends on the main objective(s) of the trial and thus on the underlying hypotheses that will be tested [1]: whether a global test should be performed or not, and whether (and how many) pairwise comparisons were planned, with statistical adjustment or not.

The question of adjusting for control type I error in multiple-arm trials is a subject of debate [2, 3, 6, 16–18]. Controlling for type I error is not needed when several experimental arms are compared with the control or the standard arms [3], but is necessary when adjusting for post hoc comparisons or when the tested hypotheses cannot be prioritized [18]. Reasons for using adjustment or not are often subjective, and should be justified [18].

Our study has several limitations. First, we assessed reports of RCTs and not protocols. This point is particularly important for assessing planned comparisons. We did not assess protocols because of the difficulties in obtaining access to trial protocols [19]. Second, the methods may have been pre-specified but not reported in the articles [20, 21]. Third, our results are limited to the core clinical journals defined by the National Library of Medicine, so our findings may not be applicable to journals outside this sample. We chose the core clinical journals because they cover all clinical and public-health areas and all major medical journals. The methodological quality of reports in other journals is unlikely to be better than in these journals.

The CONSORT (Consolidated Standards of Reporting Trials) group is developing recommendations to help improve the reporting of multiple-arm trials [22, 23]. Compared with two-arm RCTs, multiple-arm trials are more complex to design and require more complex analysis, and the results are more complex to report. The design and objectives of the trials have direct consequences for the conduct, analysis of results (for example, planned comparisons, sample-size calculation, adjustment during analysis) and reporting. The specific characteristics of multiple-arm trials and their heterogeneity in objectives, in addition to the usual requirements for reporting the results of RCTs (such as randomization, concealment, and blinding), pose a supplementary challenge for authors reporting the results of multiple-arm trials.