The same average can hide different spreads
An X-bar chart plots the average within each subgroup. An R chart plots the highest reading minus the lowest reading in that subgroup. For hypothetical packaging thickness readings of 0.99, 1.00 and 1.01 mm, the average is 1.00 mm and the range is 0.02 mm. A second subgroup of 0.95, 1.00 and 1.05 mm has the same average and a range of 0.10 mm.
Investigate a change in spread
The R chart can signal a change in within-subgroup variation while the subgroup averages remain steady. Check whether the subgroup still uses comparable parts, then investigate the measurement setup and process conditions. Mixing cavities with different typical readings can increase the range even when each cavity’s own variation is steady.
Read the range chart alongside the average chart because the usual X-bar limits are estimated from within-subgroup spread. Use the applicable product limits and decision rule to assess individual components.
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