Subjective and objective outcomes in randomized clinical trials: definitions differed in methods publications and were often absent from trial reports

Objectives: The degree of bias in randomized clinical trials varies depending on whether the outcome is subjective or objective. Assessment of the risk of bias in a clinical trial will therefore often involve categorization of the type of outcome. Our primary aim was to examine how the concepts "subjective outcome" and "objective outcome" are defined in methodological publications and clinical trial reports. To put this examination into perspective, we also provide an overview of how outcomes are classified more broadly.

Study design and setting: A systematic review of methodological publications providing a classification of clinical trial outcomes and a descriptive study of how outcomes were classified in 200 PubMed indexed clinical trial reports published in 2012.

Results: We identified 90 methodological publications with some form of a classification of outcomes. Three distinct definitions were provided for subjective outcome: (1) dependent on assessor judgment, (2) patient-reported outcome, or (3) private phenomena (ie, phenomena only assessable by the patient). Of the 200 clinical trial reports, 12 used the term "subjective" and/or "objective" about outcomes, but no clinical trial reports explicitly defined the terms.

Conclusion: The terms "subjective" and "objective" are ambiguous when used to describe outcomes in randomized clinical trials. We suggest that the terms should be defined explicitly when used in connection with the assessment of risk of bias in a clinical trial, in metaepidemiological research, and generally in the reporting of clinical trials. We also suggest that adding an explicit clarification of the terms in future versions of the Cochrane Handbook might further strengthen its important role in guiding review authors.