Credibility of claims of subgroup effects in randomised controlled trials: systematic review by Sun, X, Briel, M, Busse, JW, You, JJ, Akl, EA, Mejza, F, Bala, MM, Bassler, D, Mertz, D, Diaz-Granados, N, Vandvik, PO, Malaga, G, Srinathan, SK, Dahm, P, Johnston, BC, Alonso-Coello, P, Hassouneh, B, Walter, SD, Heels-Ansdell, D, Bhatnagar, N, Altman, DG, Guyatt, GH