Below is a summary of the search yields both with and without applying an RCT filter for a sample of five CERs.
Agency for Healthcare Research and Quality (US), Rockville (MD)
Seida JC, Dryden DM, Hartling L. Observational Studies: Empirical Evidence of Their Contributions to Comparative Effectiveness Reviews [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2013 Dec. Appendix C, Search Yields With and Without an RCT Filter.