To review systematically empirical studies of the effects of masking and not masking at the inclusion stage of a systematic review, and the influence of masking on the findings of that review.

Definitions of terminology used
As far as possible, the following terms will be used in this protocol in order to be clear about the different types of research under discussion:

• "Review": this methodology review.
• "Empirical study": a comparison of masked and unmasked trial inclusion.
• "Masked-Unmasked measure": a method of comparing masked and unmasked conditions (e.g. ratio of odds ratios).
• "Systematic review": a review of randomised trials of health care interventions.
• "Meta-analysis": statistical synthesis of the results of the trials in a systematic review.
• "Trial": a study assessed for inclusion in a systematic review.
• "Reviewer": a person making decisions about the inclusion of studies in a systematic review, using predefined eligibility criteria.
• "Summary statistics": pooled results of the meta-analyses (e.g. odds ratio, weighted mean difference) in a systematic review.