Not all of the measured data points are used to determine visual acuity. The points used are selected during analysis, and appear as larger boxes on the graph. Because the VER signal is very small measured in millionths of a volt electrical noise may easily confound the measurement. The dotted line in the lower portion of the graph shows the noise level at each measurement point. Data points that are close to or below the noise level are unreliable and are not used in determining the patient's acuity.
Linear regression is used to find the stripe size at which the relative amplitude is zero.
The linear regression fit to the acceptable data points is shown and extrapolated to zero amplitude.