High probability of special cause variation: Too few runs Too many runs = 0.05) (
Run chart analysis: Run length Special cause—run length: <20 data points (not on median): A run of 7 data points on one side of the median (either above or below) 20+ data points (not on median): A run of 8 data points on one side of the median
Run chart analysis: Trends Special cause—trends: Consecutive points all going up or all going down. May cross the median. Ignore 2+ consecutive points that are the same. (Pyzdek, 2003) 4 5 to 8 7 101 or more 6 21 to 100 5 9 to 20 # Consecutive points all increasing or decreasing Total # data points on chart
Run chart analysis: Freaks Freaks: The presence of more than one or two dramatic spikes suggests the process is out of control. Run charts not as sensitive in identifying, thus may fail to detect.
Run chart analysis: Cycling Cycling: A zigzag or saw-tooth pattern with 14+ points in a row alternating up or down.
Take on values on a continuous scale Whole numbers and decimals Can be converted to count Count observations or incidents falling into categories Whole numbers only Cannot be converted to measurement Measurement/continuous Count/attribute
Control chart example 1 Common cause variation only
Control chart example 3 Common cause variation only; can predict will stay within control limits, if no changes
Control chart example 4 Out of control, unpredictable
Just because a process is under control (common cause variation only), it does not mean that the process is meeting expectations. It just means that the process is predictable and you are getting consistent performance.