Table 1.

Definition of Visual Acuity Categories Used in the Model

Visual Acuity Category*	Corresponding Snellen Acuity†	Corresponding ETDRS Acuity†	Change in ETDRS Letters Relative to Starting Acuity of 60
1	20/30 or better	≥75	≥15-letter gain
2	20/40	70–74	10- to 14-letter gain
3	20/50	65–69	5- to 9-letter gain
4	20/60–20/100	51–64	9-letter loss to 4-letter gain
5	20/125	46–50	10- to 14-letter loss
6	20/200 or worse	≤45	≥15-letter loss

ETDRS = Early Treatment Diabetic Retinopathy Study.

^*Visual acuity categories were selected to correspond to measured clinical trial outcomes (change in ETDRS letters) (21–33).

^†The Snellen and ETDRS charts are standardized methods of measuring visual acuity. The Snellen chart is used most in practice, and the ETDRS chart is a research-quality visual acuity used for more standardized measurement in clinical trials (32, 33). These conversions are standard and well-accepted.