Medical librarian and blogger, Mark MacEachern linked to a BMJ article that looks at the accuracy of the MEDLINE Randomized Control Trial publication type. They found that not all RCT's are indexed with the matching publication type- no real surprise there.

Mark also linked to an article on creating search filters to pull up RCT's that might not be correctly indexed. It's titled, Retrieving Randomized Control Trials: A Comparison of 38 Published Search Filters. Helpful if you need to create a very thorough search.