In a sigma chart, what is the precise reason for calculating Sigma separately for each subgroup rather than calculating grand average for sigma?

OPTIONS

Sub group statistics are important to be representedSub group sigma is generally more accurateSub group sigma doesn’t introduce unnecessary special causeRange is considered better than Sigma

ANSWER

Sub group sigma doesn’t introduce unnecessary special cause

EXPLANATION

The primary for reason for doing what is specified in the question is because subgroup sigmas present the actual picture and doesn’t show unnecessary special causes.