Evaluating biases in each single study is attractive, but most difficult, since the data are usually limited, unless designs and analysis plans are registered a priori [9]. It is easier to evaluate bias across multiple studies performed on the same question. When tests of bias are applied to a wider scientific corpus, it is difficult to pinpoint which single studies in this corpus of evidence are affected more by bias. The tractable goal is to gain insight into the average bias in the field.
