Once the searches have been carried out and the studies selected for inclusion in the review, it is time to assess the quality of the studies. Quality assessment, or the critical appraisal of a study, is the evaluation of an individual study in terms of how well it has been done methodologically. A number of tools and checklists have been developed for different research designs, and it is also possible to assess the quality of published systematic reviews using tools developed for this purpose. These tools focus on those aspects that may introduce a risk of systematic error, or bias, into studies. Such errors include, for example, errors in the randomization of subjects, or inadequate outcome measurement.