7 Key SPC Tests

In Statistical Process Control (SPC), the commonly used “7 Tests” (also called Western Electric / Nelson-style rules) help detect non-random patterns in control charts—especially useful in your IMR charts or process stability analysis.

Test 1: One point beyond control limits

• Rule: Any single point outside UCL or LCL

• Meaning: Strong signal of special cause variation

• 👉 Immediate investigation required

Test 2: 7 points in a row on one side of the mean

• Rule: All 7 consecutive points are above or below the center line

• Meaning: Process shift has occurred

Test 3: 7 points in a row trending up or down

• Rule: Continuous increase or decrease

• Meaning: Systematic trend (e.g., tool wear, learning effect)

Test 4: 14 points alternating up and down

• Rule: Strict zig-zag pattern

• Meaning: Over-adjustment / tampering in process

Test 5: 2 out of 3 points beyond 2σ (same side)

• Rule: 2 of 3 consecutive points fall beyond 2 standard deviations

• Meaning: Early warning of process shift

Test 6: 4 out of 5 points beyond 1σ (same side)

• Rule: 4 of 5 consecutive points beyond 1 standard deviation

• Meaning: Moderate signal of non-random behaviour

Test 7: 15 points within 1σ of mean (both sides)

• Rule: All 15 points are very close to the mean

• Meaning: Reduced variation / possible data manipulation / stratification

Test 8: 8 points outside 1σ zone on either side of the mean

• Rule: 8 consecutive points occur outside the 1σ zone on either side of the mean

• Meaning: Points may alternate above and below the mean

SUMMARY (To Remember)

1. One point beyond 3σ

2. 7 points on same side of centreline

3. 7 points increasing/decreasing

4. 14 points alternating up/down

5. 2 of 3 points beyond 2σ

6. 4 of 5 points beyond 1σ

7. 15 points within 1σ

8. 8 points outside 1σ

Rule No: 1,5, 6, 2

Rule No: 3, 8

Rule No: 7, 4