A proportional error scales with the property being measured, so larger prediction values come with larger errors. For example, a car's speedometer, which measures wheel rotation rate and converts that to a speed by assuming a tire radius, will overestimate the car's speed by the ratio of the real tire radius to the assumed tire radius. The error will be a constant multiple of the measured speed. So, this imperfect speedometer has zero error when the speed is zero, but as the speed increases, so does the magnitude of the error. An equation of the form shown in equation 2 would be used to correct for this type of error.