To date, there has been no explicitly systematic review of the available literature on this topic. In contrast to non-systematic reviews which can be biased and incomplete, the prime purpose of systematic reviews of literature is to provide a comprehensive display of all available evidence in a common format. Systematic reviews have clear principles for their conduct. First, the process of the review should be carried out according to a pre- specified method. Second, the proposed method should be open to public scrutiny and peer review. Third, the review should be comprehensive within its pre-specified.
