The Cochrane Collaboration estimates that only "10% to 35% of medical care is based on RCTs". On what information is this estimate based?

The Cochrane Collaboration has not actually conducted research to determine this estimate; it is possible that the estimate of 10-35% comes from the following passage in a chapter by Kerr L White entitled 'Archie Cochrane's legacy: an American perspective' in the book 'Non-random Reflections on Health Services Research: on the 25th anniversary of Archie Cochrane's Effectiveness and Efficiency'. This book (published by the BMJ Publishing Group) was edited by Alan Maynard and Iain Chalmers. Iain was formerly Director of the UK Cochrane Centre, and the driving force behind the establishment of The Cochrane Collaboration; he knew Archie Cochrane well. The passage reads as follows: "In 1976 Archie [Cochrane] and I [Kerr White] were guests at several institutions in New Zealand and on one occasion we both addressed a meeting at Wellington Hospital. Not wishing to startle unduly the staid group of white coated clinicians I tempered my message slightly by stating, instead of 10-15%, that ...