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. Open the Xbar Chart dialog box.
    • PC: STATISTICS > Control Charts > Continuous > Xbar
    • Mac: Statistics > Continuous > Xbar
  3. From the first drop-down list, select Data are in one column for all subgroups. In Data column, enter Weight.
  4. From the second drop-down list, select Subgroup IDs are in a column. In Subgroup size, enter Subgroup ID.
  5. Click OK.

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.
Method
Parameters
Mean
StDev
Both parameters estimated
Tests for Special Causes
Results for Xbar Chart
Test
Description
Failed Subgroups
1 point > 3 standard deviations from the center line
3
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