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

ABSTRACT

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.

KEYWORDS:

• Causal inference
• fungal keratitis
• randomized controlled trial
• regression discontinuity

Supplemental data for this article can be accessed here.

Declaration of interest

None to report.

Meeting Presentation

Results presented in part at the 38th Annual Conference of the Society for Clinical Trials, Liverpool, United Kingdom, 9th May 2017.

Additional information

Funding

The Mycotic Ulcer Treatment Trials were supported by the National Eye Institute [Grant U10EY018573] to TML and NRA. CEO was supported in part by the National Institute of Mental Health [R25MH083620]. JRN was supported by the National Eye Institute [K23EY025025].