Were the groups in the trabeculectomy versus Ahmed valve study really comparable?

We read with interest the comparison by Tran et al of success rates of Ahmed valve implantation versus trabeculectomy in open-angle glaucoma.1 As the authors know, the retrospective nature of the study may result in selection bias. The validity of the comparison depends critically on the two groups of patients being similar preoperatively, and we feel this may not have been adequately shown. The mean baseline visual acuities (table 1) of the …