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.
➝ 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.
👉 Check this article for, Complete understanding of  Fundamentals of what is Control Chart? | Types of the Control Chart | Make your chart in easy 8 steps.



Different zones in the control chart

➝ 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.

Different Zones of Control Chart



Eight Control Chart Rules

➝ The Eight different rules are mentioned below (Source: AIAG – (SPC) 2nd Edition)
  1. One or More points are more than 3𝝈 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 > 2𝝈 from the center-line (same_side)
  6. 4 out of 5 consecutive points > 1𝝈 limit from the center-line (same_side)
  7. 15_points in a row within 1𝝈 of the center-line (either_side)
  8. A run of 8 consecutive points > 1𝝈 from the center-line (either_side)

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

→ 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

Rule 1 2 3


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 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

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 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

Rule 4 & 5



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

→ 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

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

→ 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

Rule 6



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

→ 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

Rule 7

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

→ 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

Rule 8



Summary of Control Chart Rules:

→ 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.

Summary of Control Chart Rules

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