The standard definitions of “improved vision” and “stable vision” are as given in #1966: #msg-2936366. “Severe vision loss” also has a standard definition which is a loss relative to baseline of 6+ lines (30+ letters).
For convenience, here are the standard definitions used by the FDA for all three of the terms in question:
IMPROVED VISION is a gain of 3+ lines (15+ letters) on the eye chart relative to baseline.
STABLE VISION is a gain on the eye chart of less than 3 lines (i.e. not enough to be considered “improved”), no change, or a loss of less than 3 lines (less than 15 letters) relative to baseline.
SEVERE VISION LOSS is a loss relative to baseline of 6+ lines (30+ letters).
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