RT Journal Article SR Electronic T1 Combining and converting groups when extracting data for meta-analysis JF BMJ Evidence-Based Medicine JO BMJ EBM FD BMJ Publishing Group Ltd SP bmjebm-2020-111647 DO 10.1136/bmjebm-2020-111647 A1 Kathryn S Taylor A1 Kamal R Mahtani A1 Jeffrey K Aronson YR 2021 UL http://ebm.bmj.com/content/early/2021/02/08/bmjebm-2020-111647.abstract AB