8.1  Introduction

The extent to which a Cochrane review can draw conclusions about the effects of an intervention depends on whether the data and results from the included studies are valid. In particular, a meta-analysis of invalid studies may produce a misleading result, yielding a narrow confidence interval around the wrong intervention effect estimate. The evaluation of the validity of the included studies is therefore an essential component of a Cochrane review, and should influence the analysis, interpretation and conclusions of the review.


The validity of a study may be considered to have two dimensions. The first dimension is whether the study is asking an appropriate research question. This is often described as ‘external validity’, and its assessment depends on the purpose for which the study is to be used. External validity is closely connected with the generalizability or applicability of a study’s findings, and is addressed in Chapter 12.


The second dimension of a study’s validity relates to whether it answers its research question ‘correctly’, that is, in a manner free from bias. This is often described as ‘internal validity’, and it is this aspect of validity that we address in this chapter. As most Cochrane reviews focus on randomized trials, we concentrate on how to appraise the validity of this type of study. Chapter 13 addresses further issues in the assessment of non-randomized studies, and Chapter 14 includes further considerations for adverse effects.  Assessments of internal validity are frequently referred to as ‘assessments of methodological quality’ or ‘quality assessment’. However, we will avoid the term quality, for reasons explained below. In the next section we define ‘bias’ and distinguish it from the related concepts of random error and quality.