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. Open the Xbar-S Chart dialog box.
    • PC: STATISTICS > Control Charts > Continuous > Xbar-S
    • Mac: Statistics > Continuous > Xbar-S
  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

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 the Xbar chart.

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
Results for S Chart
Test
Description
Failed Subgroups
1 point > 3 standard deviations from the center line
None
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