Zone Analysis

As described in the discussion of the Middle Third Rule, the spread between the current control limits of a mean or range chart can be divided into 6 zones each zone being one sigma wide

 

Zones C are the zones closest to the centerline; zones C+ and C- constitute the middle third region of the control chart. Zones A are the zones closest to the chart control limit. The zone between zone C and zone A comprises of Zone B. Zone Analysis is based on the Western Electric Zone Analysis method.

 

zones.gif

 

 

Zone A -

Zone A for Mean Chart –

If consecutive 2 of 3 samples lie in Zone A+ or above then trends will get raised as 2 of every 3 samples in zone A+ or above.

If consecutive 2 of 3 samples lie in Zone A- or above then trends will get raised as 2 of every 3 samples in zone A- or below

 

Zone A for Range Chart –

If consecutive 2 samples lie in Zone A+ then trends will get raised as 2 samples in a row in zone A+ or above

If consecutive 4 samples lie in Zone A- then trends will get raised as 4 samples in a row in zone A- or below

 

Zone B -

Zone B for Mean Chart –

If consecutive 4 of 5 samples lie in Zone B+ or above then trends will get raised as 4 of every 5 samples in zone B+ or above

If consecutive 4 of 5 samples lie in Zone B- or below then trends will get raised as 4 of every 5 samples in zone B- or below

 

Zone B for Range Chart –

If consecutive 3 samples lie in Zone B+ or above then trends will get raised as 3 samples in a row in zone B+ or above

If consecutive 6 samples lie in Zone B- or below then trends will get raised as 6 samples in a row in zone B- or below

 

Zone C -

Zone C for Mean Chart –

If consecutive 8 samples lie in Zone C+ or above then trends will get raised as 8 samples in a row in zone C+ or above

If consecutive 8 samples lie in Zone C- or below then trends will get raised as 8 samples in a row in zone C- or below

 

Zone C for Range Chart –

If consecutive 10 samples lie in Zone C- or below then trends will get raised as 10 samples in a row in zone C- or below

If consecutive 7 samples lie in Zone C+ or above then trends will get raised as 7 samples in a row in zone C+ or above