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FIGURE 5-1
Normal distribution curve with a small standard deviation.
same general shape. The breadth of a distribution curve (the spread of results)
is a measure of the reliability of the results. Two major ways of describing this
spread are average deviation and standard deviation.
Average Deviation
The deviation of each individual measurement (x) from the average (x) of all the
measurements is found by simple subtraction; the deviation of the /th measurement is x t - x. We are interested in the sizes of the deviations, without
regard to whether they are + or -. That is, we are interested only in the
absolute values of the deviation,
average of these absolute values:
. The average deviation is simply the
Average deviation =
x, - x
x 2 — x
FIGURE 5-2
Normal distribution curve with a large standard deviation.