Randomised controlled trials – the gold standard for effectiveness research

RCTs are prospective studies that measure the effectiveness of interventions. Although no study is likely on its own to prove causality, randomisation reduces bias and provides a rigorous tool to examine cause–effect relationships between an intervention and outcome. This is because the act of randomisation in a large study balances participant characteristics (both observed and unobserved) between the groups, allowing attribution of any differences in outcome to the intervention. This is not possible with any other study design, so RCTs are considered the reference standard for driving practice (Tarnow-Mordi et al., BJOG 2017;124:613). In designing an RCT, researchers must carefully select the participants, the interventions to be compared and the outcomes of interest (Lane BJOG 2018;125:1057; Lane BJOG 2018; 125:1504). Once these are defined, the number of participants needed to obtain reliable results is calculated (power calculation). All RCTs should have pre-specified outcomes, and should be prospectively registered with a clinical trials database to avoid selective reporting (Prior et al., BJOG 2017;124:1008–15). With appropriate ethical approvals in place, participants are then recruited and randomly assigned to either the intervention or the comparator group. It is important to ensure that at the time of recruitment there is no prior knowledge of the group to which the participant will be allocated. This is known as concealment and can be implemented using, for example, computer-generated randomisation systems. Following randomisation, RCTs can be blinded if feasible so that the participants, doctors and nurses as well as researchers do not know what treatment each participant is receiving, further minimising bias. In addition to trial registration, a full trial protocol documents full details of all trial processes before commencement (Lane BJOG 2018; 125:1504). Results of RCTs should usually be produced by intention-to-treat analysis (participants analysed in the groups to which they were randomised), as other variations, e.g. per protocol analyses (only participants who completed the treatment originally allocated are analysed) are often regarded as being biased when determining effectiveness. RCTs can have drawbacks, e.g. loss to follow up and missing data can threaten the validity of intention-to-treat analyses. Additionally, problems with generalisability can arise if participants that volunteer to participate are not similar to patients for whom RCT results will be used in the future. Expense is another criticism of RCTs, but this should be balanced against practice without RCTs, which may be even more costly. None declared. Completed disclosure of interests form available to view online as supporting information. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.