Example of a histogram with fit
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You work for a shampoo manufacturer and need to ensure that the caps on your bottles are being fastened properly. If fastened too loosely, they may fall off during shipping. If fastened too tightly, they may be hard for you customers to open (especially in the shower).

You collect a random sample of bottles and test the amount of torque required to remove the caps. Create a histogram with a fitted normal distribution to evaluate how close your samples were to the target value of 18 and whether the data are distributed normally.

1    Open the worksheet CAP.MTW.

2    Choose Graph > Histogram.

3    Choose With Fit, then click OK.

4    In Graph variables, enter Torque.

5    Click Scale, then click the Reference Lines tab.

6    In Show reference lines at data values, enter 18.

7    Click OK in each dialog box.

Graph window output

Interpreting the results

Mean torque for the sample was 21.26, slightly more than the target value of 18. Only one cap was very loose, with a torque of less than 11. However, the distribution is positively skewed and several caps were much tighter than they should be. Many caps required a torque of greater than 24 to remove and five caps required a torque of greater than 33, nearly two times the target value.

Because the sample data are so skewed, the normal distribution does not fit very well.