Bob Luttman, Robert Luttman & Associates
Individuals Control Chart
- Use
A variables chart used when the data consists of individual variables (not attributes) data points.
- Construction
+ Plot run chart
+ Calculate average and standard deviation
+ Draw horizontal line for the average
+ Draw horizontal line for the Upper Control Limit (UCL):
UCL = Average + (3 x Std. Dev.)
+ Draw horizontal line for Lower Control Limit (LCL):
LCL = Average-(3 x Std. Dev.)
if this is less than or equal to 0 then,
LCL=0
- Interpretation
+ Any point outside the control limits is due to "special cause" variation and requires investigation.
+ Individual points within the limits are a part of the normal (common cause) variation in the process. Improving them requires examination of the entire process.
+ Run chart trend analysis is also applicable. For example runs up/down.
+ 2 of 3 consecutive point above or below the 2 standard deviation limits.
+ 4 of 5 consecutive points above or below the 1 standard deviation limits.
Note: Using more tests increases the likelihood of generating a false alarm.
Individuals Control Chart - Example
Control Chart of PostOp LOS
- 3 Points are above the Upper Control Limit
- Several "runs" (consecutive points) of more than 6 below the mean. Especially at the beginning and end of the data. What appears to be happening is that the process started out OK, then went nuts in the middle before returning to normal.
This process is not in control. Something fishy is going on.
