ABSTRACT
42Given a vast quantity of heterogeneous literature, the type of items that should be collected should include the report of the study, the study itself, the patients, the research design, the effect size, and the methodological quality. In the context of a systematic review, the validity of a study is the extent to which its design and conduct are likely to prevent systematic errors or bias. The internal validity of a study is the extent to which systematic error (bias) is minimized. The external validity is the extent to which the results of the study provide a correct basis for applicability to other circumstances. There are many checklists and scales available to be used as evaluation tools. Quality assessment of individual studies that are summarized in meta-analysis is necessary to limit bias in conducting the systematic review, gain insight into potential comparisons, and guide interpretation of findings. Various sources of bias are selection bias, performance bias, attrition bias, detection bias, and time-lag bias. There are two major difficulties with assessing the validity of studies. The first is inadequate reporting of trials. The second is evidence of a relationship between parameters thought to measure validity and actual study outcomes.
