Determination of valid benchmarks for outcome indicators in cataract surgery: a multicenter, prospective cohort trial - Hahn, U; Krummenauer, F; Kölbl, B; Neuhann, T; Schayan-Araghi, K; Schmickler, S; von Wolff, K; Weindler, J; Will, T; Neuhann, I (Ophthalmology, 2011)