The Normal Distribution
From the illustration in figure A-2, it can be seen that the normal distribution curve is bell shaped, with a high point in the middle and an everdecreasing slope toward horizontal at the ends.
Figure A-2. Normal distribution curve.
One of the important things about the normal distribution is that the formula for calculating it depends on only two factors, the mean value and the standard deviation. The mean value locates the middle of the curve, or the peak. The standard deviation shows whether the curve is clustered tightly around the midpoint or whether it is loosely clustered (figure A-3).