Use S charts to determine whether or not the process variation is in control. S charts can detect changes in the process standard deviation by monitoring the differences within subgroups.

·    An in-control process exhibits only random variation within the control limits. If the subgroup standard deviations do not fall outside the control limits, and if there is no evidence of nonrandom variation within the control limits, then the process is considered to be in control with respect to variability.

·    An out-of-control process exhibits unusual variation, which may due to the presence of special causes. If subgroup standard deviations fall outside the control limits or if there is evidence of nonrandom variation within the limits, then the process is out of control with respect to variability.

A canning company wants to assess whether or not its can-filling process is in control. Fifteen subgroups of ten cans each were collected at fifteen-minute interval across two shifts for a single day's production.

To minimize the within-subgroup (can-to-can) variation the ten samples for a given subgroup were gathered in a short period of time. Minimizing this variation is important because within-subgroup variation is used to establish the control limits for the -S Chart.

Data: Canning.MTW (available in the Sample Data folder).