Example of Normality Test
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In an operating engine, parts of the crankshaft move up and down. AtoBDist is the distance (in mm) from the actual (A) position of a point on the crankshaft to a baseline (B) position. To ensure production quality, a manager took five measurements each working day in a car assembly plant, from September 28 through October 15, and then ten per day from the 18th through the 25th.

You wish to see if these data follow a normal distribution, so you use Normality test.

1    Open the worksheet CRANKSH.MTW.

2    Choose Stat > Basic Statistics > Normality Test.

3    In Variable, enter AtoBDist. Click OK.

Graph window output

Interpreting the results

The graphical output is a plot of normal probabilities versus the data. The data depart from the fitted line most evidently in the extremes, or distribution tails. The Anderson-Darling test's p-value indicates that, at a levels greater than 0.022, there is evidence that the data do not follow a normal distribution. There is a slight tendency for these data to be lighter in the tails than a normal distribution because the smallest points are below the line and the largest point is just above the line. A distribution with heavy tails would show the opposite pattern at the extremes.