Example of a dotplot with stack y's
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Your company makes plastic pipes, and you want to test the consistency of their diameters. You measure 10 pipes a week for 3 weeks. Create a dotplot to display the results.

1    Open the worksheet PIPE.MTW.

2    Choose Graph > Dotplot.

3    Choose Multiple Y's - Stack Y's, then click OK.

4    In Graph variables, enter 'Week 1'  'Week 2'  'Week 3'. Click OK.

Graph window output

Interpreting the results

Most diameters range from about 4.7 to 5.8. However, several diameters (one from Week 2 and four from Week 3) are greater than 6.4. One diameter from Week 3 is close to 9.0. Variability in pipe diameters appears to increase from week to week, and the means may also be increasing.

You may want to use ANOVA to compare the means. Before using ANOVA, you can use Stat >ANOVA > Test for Equal Variances to test if the variances are significantly different.