Example of an interval plot with multiple y's and groups
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Your company makes plastic pipes and you are concerned about the consistency of the diameters. You measure ten pipes each week for three weeks. Create an interval plot to examine the distributions.

1    Open the worksheet PIPE.MTW.

2    Choose Graph > Interval Plot or Stat > ANOVA > Interval Plot.

3    Under Multiple Y's, choose With Groups. Click OK.

4    In Graph Variables, enter 'Week 1'  'Week 2'  'Week 3'.

5    In Categorical variables for grouping (1-3, outermost first), enter Machine.

6    Under Scale Level for Graph Variables, choose Graph variables displayed innermost on scale. Click OK.

Graph window output

Interpreting the results

The interval plots shows:

·     For machine 1, the means and confidence intervals appear to drift upward each week.

·     For machine 2, the means and confidence intervals appear to be much more consistent across weeks.

You conclude that the variance may be related to the mean. You may want to perform other statistical analyses, such as balanced MANOVA, to further examine the relationship between factors.