Single screening or double screening for study selection in systematic reviews?

​Recent systematic reviews provide evidence on the need for more than one reviewer when screening studies for inclusion in systematic reviews . Systematic reviews are important tools for the practice of evidence-based healthcare. Part of their value comes from their ability to obtain new knowledge by synthesising existing data. The reliability of this knowledge arises from the review process, which, when implemented correctly, should be predefined, transparent and rigorous.1 Conducting a high-quality systematic review has resource implications, for example, time, size of the research team and costs. A random sample of systematic reviews in the Prospective Register of Systematic Reviews (PROSPERO) showed that the mean estimated time to complete the project and publish the review was 67 weeks (SD=31; range 6–186 weeks); each review needed a mean of 5 team members (SD=3; range 1–27).2 The search for publications, review of abstracts and retrieval and review of selected full-text papers alone can take an average of 332 hours.3 Systematic reviews are not cheap; some estimates suggest that they can cost several hundred thousand pounds.4 The need to balance timeliness of results with rigour has meant that consideration has been given to ways in which the review process could be restricted, for example, by reducing the number of reviewers who screen studies for inclusion …