Not only randomised controlled trials, but also controlled observational studies.
Leiden University Medical Centre - Gynaecology and Clinical Epidemiology , Leiden , the Netherlands . E-mail: F.M.Helmerhorst@LUMC.nl.
Frans M
FM
Helmerhorst
D019317:Evidence-Based Medicine / Q000592:standards*; D006801:Humans; D015201:Meta-Analysis as Topic; D012196:Review Literature as Topic