Navigating blinding challenges in complex intervention trials: insights from a UK researcher survey

One major methodological challenge faced by researchers when conducting randomised controlled trials (RCTs) of complex interventions is blinding. Given the nature of complex interventions, trial participants and intervention providers often cannot be blinded to the intervention. This lack of blinding may result in a lower evidence rating for the trial, potentially hindering the implementation of its findings. One option to mitigate this issue is to single blind studies, with the blinding only of outcome assessors to group allocation. Despite the potential advantages, outcome assessment blinding is frequently underutilised. The challenges behind its limited adoption in this field are not clearly understood. This study aimed to explore the attitudes and practices of UK researchers regarding blinding, particularly outcome assessment blinding, in complex intervention RCTs.

We conducted an online survey targeting researchers affiliated with the UK Clinical Research Collaboration (UKCRC) Clinical Trials Units (CTUs), the National Institute for Health and Care Research (NIHR), Trials Methodology Research Partnership (TMRP) and National Health Service (NHS)-NIHR research units.

A total of 63 researchers participated in the survey. Most were affiliated with UKCRC (49/63, 77%), TMRP (6/63, 8%) or NHS-NIHR research units (8/63, 12%). The majority (57/63, 91%) agreed that complex interventions pose significant challenges to adequate blinding. Concerns about compromised internal validity due to lack of blinding were noted by 45% of respondents (28/63), and 67% (42/63) expressed dissatisfaction with existing quality assessment tools for complex intervention RCTs. Although 66% of respondents (41/63) found outcome assessment blinding often feasible, 52% (33/63) identified limited resources as a primary obstacle. Free-text responses highlighted practical constraints and additional costs associated with blinding. Furthermore, two-thirds of respondents (43/63, 68%) reported a lack of specific recommendations on blinding, which may exacerbate these challenges. Opinions on the influence of the new Medical Research Council (MRC)-NIHR Framework 2021 were mixed. However, most respondents 76% (48/63)) agreed that NIHR plays a role in improving blinding strategies in complex intervention RCTs and 79% (50/63) agreed that a wider stakeholder engagement is necessary to overcome blinding challenges.

This survey offers insights into the views of experienced UK-based researchers on blinding in complex intervention RCTs. Four key findings emerged: (1) the challenges of blinding due to the inherent complexity of these interventions, (2) the essential role of blinding in reducing bias, (3) the variability in the feasibility of outcome assessment blinding due to practical and resource constraints and (4) the positive impact of stakeholder involvement in addressing blinding challenges. However, respondents expressed dissatisfaction with current quality assessment tools and held divergent views on outcome assessment blinding, highlighting the need for more definitive methodological guidance on blinding in complex intervention trials.