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 represented
• Sub group sigma is generally more accurate
• Sub group sigma doesn’t introduce unnecessary special cause
• Range is considered better than Sigma

ANSWER

Sub group sigma doesn’t introduce unnecessary special cause

EXPLANATION

Sub group sigma doesn’t introduce unnecessary special cause – It is this reason why Sigma is calculated for each subgroup rather than calculating the grand sigma.