Retinitis pigmentosa (RP) is a hereditary ocular disease in which patches of pigment appear on the retina, potentially resulting in substantial vision loss and in some cases complete blindness. An important issue is how fast the subjects decline. Visual fields is an important measure of area of vision which is measured in degree ^ 2. The visual field for normal person is around 11,000 degrees ^ 2. The longitudinal data in table 11.31 were provided by an individual patient. Suppose the rate of change of In (visual field) is a linear function of follow-up time.

problem:

What does the intercept mean in this context? What is the estimated percent decline in visual field per year?