Control Chart Rules, Patterns and Interpretation

Control Chart Rules, Patterns, and Interpretation

➝ Control Chart Rules, Patterns, and Interpretation are helping us to identify the special cause of variation from the process.
➝ So we can eliminate this cause of variation and make our process smooth.


Eight Rules of Control Chart

➝ The Eight different rules of control chart patterns are mentioned below (Source: AIAG – Statistical Process Control (SPC) 2nd Edition)
  1. One or More points are more than 3 sigmas from the center line
  2. 7 points in a row on the same side of the center
  3. 6 points in a row increasing or decreasing steadily
  4. 14 points continuously alternating up and down
  5. 2 out of 3 consecutive points > 2-sigma from the centerline (same side)
  6. 4 out of 5 consecutive points > 1-sigma limit from the center line (same side)
  7. 15 points in a row within 1 sigma of the center line (either side)
  8. A run of 8 consecutive points > 1 sigma from the center line (either side)
➝ Control Chart is a statistical tool used to differentiate between process variation resulting from a common cause & special cause.
➝ Check this article for, What is Control Chart? | Types of the Control Chart | Run Chart


➤ Different zones in the control chart

➝ Now, first of all, we define the zone for understanding the Control Chart Patterns, Rules and Interpretation for special cause of variation available.
➝ Please show the below picture for a clear understanding of UCL, LCL, Centre line and different Zone A, Zone B and Zone C.
➝ Each zone has a distance of one sigma.
➝ From center line add one sigma for each zone.

Different zones in the control chart


➤ Rule 1: One or More points are more than 3 sigmas from the center line

→ It is also called as a point beyond the control 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
→ Process step skipped
→ Process step not completed
→ Power failure
→ Equipment breakdown
Rule 1,2&3 of control chart for Special Cause

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

→ It is known as a Small shift from the average.
→ If we get continuously seven points on the same side of the center line 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


➤ Rule 3: 6 points in a row steadily increasing or decreasing

→ 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)

➤ Rule 4: 14 points continuously alternating up and down

→ This rule is also known as an over control.
→ 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
Rule 4&5 of control chart for Special Cause


➤ Rule 5: 2 out of 3 consecutive points > 2-sigma from the centerline (same side)

→ It is also known as a large shift from the average.
→ If we get 2 points out of consecutive 3 points available in >2-sigma distance from the centerline on the same side then the special cause is available in the process.
→ In other words, 2 out of 3 consecutive points are available in zone A (same side of the centerline).

➥ Possible Causes

→ The new person doing the job
→ Wrong setup
→ Measurement error
→ Process step skipped
→ Process step not completed
→ Power failure
→ Equipment breakdown

➤ Rule 6: 4 out of 5 consecutive points > 1-sigma limit from the center line (same side)

→ It is known as a Small shift from the average.
→ If we get 4 points out of consecutive 5 points available in >1-sigma distance from the centerline on the same side then the special cause is available in the process.
→ In other words, 4 out of 5 consecutive points are available 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

Rule 6 of control chart for Special Cause


➤ Rule 7: 15 points in a row within 1 sigma of the center line (either side)

→ This rule is also known as stratification.
→ If we get 15 points in a row within 1 sigma of the centerline or within the zone C (any side of control limit) 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

Rule 7 of control chart for Special Cause

➤ Rule 8: A run of 8 consecutive points > 1 sigma from the center line (either side)

→ This rule is also known as a mixture.
→ If we get 8 consecutive points > 1 sigma from the center line (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

Rule 8 of control chart for Special Cause

Summary of Rules for Special Cause

→ 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 is mentioned in the below table.

Summary of Rules for Special Cause

Related Article:

👉 SPC | Statistical Process Control

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