Minitab Cookbook by 未知

Author:未知
Language: eng
Format: epub
Publisher: Packt Publishing

Click on OK.

Select the I-MR options button, then choose the tab labeled Tests, and select Perform all tests for Special causes.

Click on OK in each dialog box.

How it works…

Each column entered into the variables section for I-MR charts will create a separate control chart. We have created charts for both the mean maximum temperatures and the mean minimum temperatures for January from the start of the data in 1853.

Results that break the rules for identifying special causes or unusual variation are flagged in red with the test that has been broken. In the results shown in the preceding screenshot, Test 1 is flagged up for years 1917 and 1963 on the moving range chart. This seems to indicate a large change in temperatures from 1916 to 1917 and 1962 to 1963. Notice the result in the individuals' chart for 1963. This corresponds to one of the coldest winters in the UK since records began.

Some care should be taken with the interpretation of the previous results as adjacent points are one year apart.

The use of the Scale option, Stamp, allows us to use a column in the worksheet for the x axis labels. By displaying the year instead of the row number as the index, we can identify the years which contain unusual results. If the graph is the active window pop up, text highlighting the year will be displayed when we hover the cursor over a point.

I-MR charts, like most control charts, are an updating graph within Minitab. We can right-click on the chart and click on Update Graph Now when new data is entered or click on Update Graph Automatically.

Time-weighted charts are more appropriate than I-MR charts if we are interested in observing small changes. Advanced charts such as CUSUM or EWMA can be used to pick up on these smaller shifts in the process. See the example of the EWMA chart later as a comparison.

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