Example of Xbar-S Chart

A quality engineer at a canning company assesses whether the can-filling process is in control. Each hour, the engineer collects a subgroup of 10 cans. To minimize the within-subgroup (can-to-can) variation, the engineer collects the cans for a given subgroup in a short period of time.

The quality engineer creates an Xbar-S chart to monitor the weight of the cans.

  1. Open the sample data, CanWeight.MTW.
  2. Choose Stat > Control Charts > Variables Charts for Subgroups > Xbar-S.
  3. From the drop-down list, select All observations for a chart are in one column, enter Weight.
  4. In Subgroup sizes, enter Subgroup ID.
  5. Click Xbar-S Options.
  6. On the Tests tab, select 1 point > K standard deviations from center line (Test 1), K points in a row on same side of center line (Test 2), and K points in a row within 1 standard deviation of center line (either side) (Test 7).
    If you are not sure which tests apply in your specific situation, use Tests 1, 2, and 7 when you first establish the control limits based on your data. After the control limits are established, you can use the known values of those limits and Test 7 is no longer needed.
  7. Click OK in each dialog box.

Interpret the results

Interpret the S chart first. None of the points are outside the control limits, and the points display a random pattern. Thus, the process variation is in control and the engineer can examine the process center on the Xbar chart.

One points fails test 1 (one point more than 3 standard deviations from center line) on Xbar chart.

Xbar-S Chart of Weight

Test Results for Xbar Chart of Weight

TEST 1. One point more than 3.00 standard deviations from center line. Test Failed at points: 3 * WARNING * If graph is updated with new data, the results above may no longer be correct.
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