The measure of dispersion is very important to special educators because they must determine the degree or amount of differences exhibited by individuals in behaviors, skills, or traits, the differentiated method must be employed in calculating differences from the average or expected score. 

This uses standard deviation, range, variance, statistical dispersion to expand or squeeze scores to find the typical score of what is being measured.

Standard Deviation 

In normal distribution,Standard deviations represent percentages  of scores shown on the bell curve. 

Note : to clarify the significance of standard deviation, it is helpful to remember that one criterion for the diagnosis of mental retardation is an IQ score of more than two standard deviation below the mean as shown above.

Skewed Distribution 

Distribution can be skewed in a positive or negative direction. When many of the scores are below the mean, the distribution is said to be positively skewed. When a large number of the scores occur above the mean, the distribution is said to be negatively skewed.