To Determine Special Causes
Plot at least 20 data points.
Special causes of variation:
- data points outside the control limits (usually +/- 3 Standard Deviations).
- A run of 8 or more data points in a row either above or below the center line.
- 6 or more increasing or decreasing in a trend.
- 3 consecutive data points near outer limits.
All other data are treated as common cause of variation.