2.3. Quality Assessment

Each of the included studies were independently assessed by two authors for quality using either the Newcastle-Ottawa scale (NOS) or the National Institute of Health (NIH) quality assessment tool). The NOS was employed in assessing the case-control (retrospective) or the cohort studies. It comprises three main parameters: selection, comparability and exposure (for case control studies)/outcome (for cohort studies). The case series studies, on the other hand, were assessed using the nine-parameter NIH tool. Any disagreement between the review authors with regards to the risk of bias in any of the studies was settled by discussion. The rating and overall score for each study are presented in a table. Each tool scores a study as either low quality (<5), moderate quality (5–7) or good quality (>7).