ABSTRACT
In this appendix, some rules for interpreting the standard deviation in relation to the normal curve are described. In addition, the computation of the standard deviation is considered.
As indicated in Topic 55, going out one standard-deviation unit on both sides of the mean captures 68% of the cases in a normal distribution. It turns out that by going out two units on both sides of the mean captures approximately 95% of the cases.1 For the distribution considered in Topic 55 with M = 70 and S = 10, it was noted that 68% lie between scores of 60 and 80. Going out 2 × 10 = 20 points (which is two standarddeviation units), scores of 50 and 90 (i.e., the mean of 70 minus 20 = 50, and the mean of 70 plus 20 = 90) are reached. This is illustrated in Figure 1.
