ArticleBMJ (Clinical research ed.)2026
Outcome switching in cohort studies of interventions: meta-epidemiological study.
Article in BMJ (Clinical research ed.), 2026. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Not yet cited in PubMed.
What it found
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The abstract states no effect estimate the extractor could read, or names no intervention and outcome on the map, so this paper lights no cell and moves no belief. It is still indexed, cited and linked below.
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Authors and funding
7 authors.
Funding
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Abstract
objectivesTo study the prevalence and characteristics of outcome switching, the completeness of outcome prespecification, and factors associated with outcome switching in observational cohort studies of interventions.
designLongitudinal meta-epidemiological study.
settingRegistry records and journal publications.
participantsControlled cohort studies investigating the effects of interventions. Eligible studies were registered on ClinicalTrials.gov within one month of their start date (2014-16) and had published results in peer reviewed journals by 2024. MAIN OUTCOMES MEASURES: Firstly, proportion of studies with outcome switching identified by comparing the prespecified outcomes in the registry and those reported in the journal publication of results. Discrepancies were categorised as omission (prespecified primary outcomes not reported), downgrading (prespecified primary outcomes reported as non-primary), upgrading (prespecified non-primary outcomes reported as primary), and introduction of new primary outcomes (not registered as an outcome). Secondly, proportion of studies with completely prespecified primary outcomes, defined as registry entries that include the measurement variable, analysis metric, method of aggregation (the statistic summarising the outcome within each study group), and time point.
resultsOf 9965 registration records screened, 124 eligible studies with results published between 2015 and 2024 were included. Only 30 studies (24%) completely prespecified their primary outcomes. Outcome switching occurred in 60 (48%) studies, but only two provided an explanation. The most common types of switching were omission (n=32, 26%) and downgrading (n=32, 26%), followed by the introduction of new primary outcomes (n=25, 20%), and upgrading (n=2, 2%). Among 57 studies with outcome switching other than omission (ie, outcome results were reported), statistically significant results were favoured in 77% (44/57) by introducing or upgrading a new significant primary outcome or downgrading a non-significant one. No study characteristics were significantly associated with outcome switching in multivariable logistic regression.
conclusionsOutcome switching and inadequate outcome prespecification were common in cohort studies of interventions. Most changes were unexplained and favoured statistically significant results, raising concerns about potential selective reporting and highlighting the need for improved transparency in outcome reporting. STUDY REGISTRATION: Open Science Framework (https://osf.io/xn5zt/).
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