Regression Discontinuity and Randomized Controlled Trial Estimates: An Application to The Mycotic Ulcer Treatment Trials

Purpose: We compare results from regression discontinuity (RD) analysis to primary results of a randomized controlled trial (RCT) utilizing data from two contemporaneous RCTs for treatment of fungal corneal ulcers. Methods: Patients were enrolled in the Mycotic Ulcer Treatment Trials I and II (MUTT I & MUTT II) based on baseline visual acuity: patients with acuity ≤ 20/400 (logMAR 1.3) enrolled in MUTT I, and >20/400 in MUTT II. MUTT I investigated the effect of topical natamycin versus voriconazole on best spectacle-corrected visual acuity. MUTT II investigated the effect of topical voriconazole plus placebo versus topical voriconazole plus oral voriconazole. We compared the RD estimate (natamycin arm of MUTT I [N = 162] versus placebo arm of MUTT II [N = 54]) to the RCT estimate from MUTT I (topical natamycin [N = 162] versus topical voriconazole [N = 161]). Results: In the RD, patients receiving natamycin had mean improvement of 4-lines of visual acuity at 3 months (logMAR −0.39, 95% CI: −0.61, −0.17) compared to topical voriconazole plus placebo, and 2-lines in the RCT (logMAR −0.18, 95% CI: −0.30, −0.05) compared to topical voriconazole. Conclusions: The RD and RCT estimates were similar, although the RD design overestimated effects compared to the RCT.