If you have a spreadsheet on your computer, you may be able to use it to compute the SD and to sort lists. Check your program. The normal curve, pictured on page 74, is the most frequenty used curve in statistics. It is centered at zero. The units on the horizontal axis are called standard units .
Standard units measure the number of standard deviations away from the average.
Carefully study the examples on page 77 to 79, and work exercise set B on page 80. Finding areas under the normal curve is a key lesson objective. Again, many spreadsheets will do these calculations automatically. Details can be found in your spreadsheet manual. To find the area between two points, you first find the total area to the left of the upper point, and subtract from it the area to the left of the lower point, leaving the area between the points.