A normal distribution is associated with the perfectly bell shaped curve you can see in statistics books. The area under a normal curve is 1.0, and what makes a normal curve look like it does is the dense concentration of scores under the highest point (the mean) tapering out to the tails where there is less density. Normal distributions are defined by 2 parameters. The mean, or the sum of all scores divided by the number of scores, and the standard deviation. In a perfectly normal distribution half of the scores will fall to the right of the mean, and half of the scores will fall to the left. Furthermore, 68% of the scores will fall within one standard deviation of the mean, 95% will fall within 2 standard deviations and 99.7% will fall within 3 standard deviations. This is called the empirical rule.