An integral component for a large tractor model has a USL of $950$mm and an LSL of $949\mathrm{.}90$mm$.$ Assuming this follows a six$-$sigma process and the standard deviation is $0\mathrm{.}0128.$

When variation in the process is observed to be abnormal, what would this indicate?

When process variation is deemed to be normal, we can attribute this to common cause variation. However, when the variation is abnormal, this indicates that there must be some culpable assignable cause variation.