The Bell Curve: The Normal Distribution

Return to Infant Toddler Sensory Processing Explanation of Scores

picture of bell curve

Many biological, psychological and social phenomena occur in the population in the distribution we call the bell curve (Portney & Watkins, 2000). This picture shows a symmetrical bell curve, in which the average score [i.e., the mean] would be plotted in the middle of the curve, where the ‘bell’ shape is the tallest. Most of the people [i.e., 68% of them, or 34% + 34%] have performance within 1 segment [i.e., a standard deviation] of the average score.

For the Sensory Profile measures, ‘typical performance’ is the same as the middle sections on the bell curve, or 68% of the population of persons without disabilities.

As we move farther away from the average performance, less and less people will have this higher or lower performance. As you can see on the diagram, approximately 16% [i.e., 14% + 2%] of the people will be higher than the average and 16% will be lower than the average.

For the Sensory Profile measures, ‘definitely more than others’ and ‘definitely less than others’ represents the 2% sections at the extremes on the bell curve. This is why we can say ‘definitely’---not very many people will perform at these levels.

For the Sensory Profile measures, ‘probably more than others’ and ‘probably less than others’ represents the 14% sections to the right and left of the bell curve.