# 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.

### ➤ 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 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 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 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 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

## 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.

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