Does question order affect alcohol outcome measurement in the ORBITAL COS?

What is it about?

The ORBITAL core outcome set (COS) specifies what outcomes should be measured in alcohol brief intervention trials, but leaves open the question of how those outcomes should be presented to participants. Question order effects are a well-documented problem in survey research: the order in which questions are asked can influence responses, potentially introducing systematic bias into outcome measurement. This study, the QOBCOS trial, recruited adults aged 18 and over who were searching online for alcohol-related help and invited them to complete the ORBITAL COS. Using a randomised factorial design, participants were allocated to different orderings of the COS questions to estimate whether and to what extent the sequence in which outcomes are presented affects the responses obtained. The study was conducted in a digital, self-administered format reflective of how the COS would be used in online alcohol brief intervention trials.

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Why is it important?

Establishing a core outcome set solves the problem of what to measure but immediately raises the practical question of how to measure it consistently. If question order produces meaningfully different responses, then trials using the ORBITAL COS in different orders would still generate incomparable results, undermining the very standardisation the COS was designed to achieve. This study directly addresses that implementation question, providing empirical guidance for researchers designing trials that incorporate the ORBITAL COS. It forms a natural companion to the broader ORBITAL programme of work and represents the kind of methodological follow-through that transforms a consensus-derived outcome framework into a reliably deployable research instrument. It also reflects the growing international uptake of the ORBITAL COS in active trial programmes.