Control Chart Rules, Patterns, and Interpretation

Different zones in the control chart

Eight Control Chart Rules

One or More points are more than 3𝝈 from the center-line 7_points in a row on the same side of the center 6_points in a row increasing or decreasing steadily 14_points continuously alternating up and down 2 out of 3 consecutive points > 2𝝈 from the center-line (same_side) 4 out of 5 consecutive points > 1𝝈 limit from the center-line (same_side) 15_points in a row within 1𝝈 of the center-line (either_side) A run of 8 consecutive points > 1𝝈 from the center-line (either_side)

Rule 1: One or More points are > 3𝝈 from the centerline

Rule 2: 7_points in a row on the same side of the center

Rule 3: 6_points in a row steadily increasing or decreasing

Rule 4: 14_points continuously alternating up and down

Rule 5: 2 out of 3 consecutive points > 2𝝈 (same_side)

Rule 6: 4 out of 5 consecutive points > 1𝝈 (same side)

Rule 7: 15_points in a row within 1𝝈 (either side)

Rule 8: A run of 8 consecutive points > 1𝝈 (either side)

Summary of Control Chart Rules:

➝ Control Chart Rules, Patterns, and Interpretation are helping us to identify the special cause of variation from the process.➝ By referring to these 8 rules, we can identify and eliminate the cause of variation and make our operation smooth.➝ It is a statistical tool used to differentiate between process variation resulting from a common cause & special cause.for, Complete understanding of Fundamentals of what is Control Chart? | Types of the Control Chart | Make your chart in easy 8 steps.➝ Now, first of all, we define the zone for understanding the Control Chart Rules.➝ Please show the below picture for a clear understanding of UCL, LCL, Centreline and different Zone A, Zone B, and Zone C.➝ Each zone has a distance of one sigma.➝ From the center-line add one sigma for each zone.➝ The Eight different rules are mentioned below (Source:→ It is also called as a point beyond the central line.→ Sometimes, Rule 1 also known as a large shift from the average.→ If we can get any point beyond the control line then the special cause is available.➨ Possible Causes:⇢ The new person doing the job⇢ Wrong setup⇢ Measurement error⇢ Operation step skipped or not completed⇢ Power failure⇢ Equipment breakdown→ It is known as a Small shift from the average.→ If we get continuously 7_points on the same side of the centerline then the special cause is available in the process.➨ Possible Causes:⇢ Raw material change⇢ Change in work instruction⇢ Different measurement device/calibration⇢ Different shift⇢ A person gains greater skills in doing the job⇢ Change in the maintenance program⇢ Change in the setup procedure→ It is known as a trend pattern.→ If we get a pattern of 6_points continuously in increasing order or a decreasing order then the special cause is available in the process➨ Possible Causes:⇢ Tooling wear⇢ Temperature effects (cooling, heating)→ This is also known as an over control pattern→ If we get 14_points in a row up and down then the special cause is available in the process.➨ Possible Causes:⇢ Tampering by operator⇢ Alternating raw materials→ It is also known as a large shift from the average.→ If we get 2_points out of consecutive 3_points present in >2-sigma distance from the center-line on the same side then the special cause is present in the process.→ In other words, 2 out of 3 consecutive points are present in zone A (same_side of the center-line).➨ Possible Causes:⇢ The new person doing the job⇢ Wrong setup⇢ Measurement error⇢ Operation step skipped or not completed⇢ Power failure⇢ Equipment breakdown→ It is known as a Small shift from the average.→ If we get 4_points out of consecutive 5_points present in >1-sigma distance from the centerline on the same side then the special cause is present in the process.→ In other words, 4 out of 5 consecutive points are present in Zone B or beyond (same side of the centerline).➨ Possible Causes:⇢ Raw material change⇢ Change in work instruction⇢ Different measurement device/calibration⇢ Different shift⇢ A person gains greater skills in doing the job⇢ Change in the maintenance program⇢ Change in the setup procedure→ This is also known as the stratification nature of the pattern.→ If we get 15_points in a row within 1 sigma of the centerline or within zone C (any side of the central line) then the special cause is available.➨ Possible Causes:⇢ Data mixed with more than one process.⇢ Mixed data of more than one shifts, machines or raw materials→ This is also known as a mixture property of pattern.→ If we get 8 consecutive points > 1 sigma from the centerline (either side) then the special cause is available.→ In other words, 8 consecutive points with no points available in zone C (any side of the centerline).➨ Possible Causes:⇢ Data mixed with more than one process.⇢ Mixed data of more than one shifts, machines or raw materials→ It is very difficult to list out all possible special_causes for each pattern.→ Because special_causes are very dependent on the type of process.→ Manufacturing processes have different issues and service-related processes have different issues.→ For, understanding purpose, some possible causes which are responsible for the special_cause of variation and identified by chart pattern are mentioned in the below table.Related Articles: