Definitions of quality validity and bias

Quality, when referring to randomised controlled trials (RCTs), is a multidimensional concept that includes appropriateness of design, conduct, analysis, reporting and its perceived clinical relevance.1-4 Validity refers to the extent to which the study results relate to the "truth". Validity may be internal (i.e. are the results of this trial true?) or external (to what extent do the results of this trial apply to my patients?). Factors affecting external validity are discussed further in Chapter 12. Internal validity is a prerequisite for external validity.

In addition to assessing the role of chance, a crucial component in appraising the internal validity of a trial is assessment of its potential for bias. Bias denotes a systematic error resulting in an incorrect estimation of the true effect. With respect to clinical trials, bias may be best understood in terms of:

• selection bias - resulting in an imbalance in treatment groups

• performance bias - treating one group of people differently from the other

• detection bias - biased assessment of outcome resulting from lack of blinding

• attrition bias - biased handling of deviations from the study protocol and those lost to follow up.

This chapter guides the reader on applying the various forms of bias to appraising the internal validity of an RCT.

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