Two studies of systematic reviews that included NRS have commented that only a minority of reviews assessed the methodological quality of included studies (Audige 2004, Golder 2006a). Members of the NRSMG have gained experience of trying to assess risk of bias in non-randomized studies. Anecdotally, review authors have reported that NRS are generally of poor methodological quality, or are poorly reported so that assessing methodological quality and risk of bias consistently across primary studies is difficult or impossible (Kwan 2004). Even the Newcastle-Ottawa scale has been reported to be difficult to apply, so agreement between review authors is likely to be modest. Methodological information can be difficult to find in papers, making the task frustrating, especially when using some of the more detailed instruments; review authors may spend a long time searching for details of what researchers did, only to conclude that the information was not reported. Nevertheless, collecting some factual information (for example, the confounders considered and what researchers did about confounding) can still be useful since such information illustrates the extent of heterogeneity between studies.