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8.12.1  Rationale for concern about bias

Several types of people can be blinded in a clinical trial: see Box 8.11.a. The second of two domains in the tool that specifically address blinding focuses on blinding of outcome assessors. If people who determine outcome measurements are aware of intervention assignments, bias could be introduced into these assessments. Outcome assessments may be made by the participants themselves, by their healthcare providers, or by independent assessors.


In empirical studies, lack of blinding in randomized trials has been shown to be associated with more exaggerated estimated intervention effects, by 9% on average, measured as odds ratio (Pildal 2007). These studies have dealt with a variety of outcomes, some of which are objective. The estimated effect has been observed to be more biased, on average, in trials with more subjective outcomes (Wood 2008).


All outcome assessments can be influenced by lack of blinding, although there are particular risks of bias with more subjective outcomes (e.g. pain or number of days with a common cold). It is therefore important to consider how subjective or objective an outcome is when considering blinding. The importance of blinding and whether blinding is possible may differ across outcomes within a study. Seemingly objective assessments, e.g. doctors assessing the degree of psychological or physical impairment, can also be somewhat subjective (Noseworthy 1994).


Blinding of outcome assessment can be impossible (e.g. when patients have received major surgery). However, this does not mean that potential biases can be ignored, and review authors should still assess the risk of bias due to lack of blinding of outcome assessment for all studies in their review.