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CR: Making an X-bar chart with a single data column

Revision Date: 2005-09-19

Problem:

Several customers have presented us with data in the following general form:

Date                 Number of days
8/1/2003          12
8/1/2003          11
8/1/2003          9
8/2/2003          8
8/2/2003          10
8/3/2003          11
8/3/2003          9
8/4/2003          8
8/4/2003          10
8/4/2003          7

They want to use this data for an X-bar chart but an X-bar chart requires subgroups of data. A subgroup is two or more numbers (observations) which are used to calculate an average.

Solution:

In this case, subgroups must be created by using multiple rows of data. We have a date we can use for grouping. You might think about this same data set in a different form like this:

8/1/2003          12, 11, 9
8/2/2003          8, 10
8/3/2003          11, 9
8/4/2003          8,10,7

Since the data is not in this handy form, you need to use a CHARTrunner feature called advanced row selection. On the data definition tab, click on the button named advanced row selection.

Unzip the attached file into a temporary folder. Run CHARTrunner and set the working folder to this temporary folder. Study the chart definition and the Excel sheet to see the solution to this problem. Since subgroup size will vary in an unpredictable way, the control limits are adjusted to reflect the changing subgroup size.

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