Example of Xbar 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 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.
  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 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

One point is out-of-control on the Xbar chart. The engineer concludes that the process is not stable and should be improved.

Xbar Chart of Weight

Xbar 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