Bob Luttman, Robert Luttman & Associates
P Chart
- Use
An attribute control chart that monitors percentages. Some healthcare related examples are % Unplanned Returns to the O.R. and % C/Section Deliveries.
- Construction
1. Obtain at least 20 samples (of variable size ni)
2. Calculate the relevant percentage for each subsample (p=Sum(Xi) / ni )
3. Calculate and plot:
p-bar = total number of units having the characteristic / total number of units
sp =
UCL = p-bar + (3 x sp)
LCL = p-bar - (3 x sp) , or 0 if LCL<0
Note: Control limits are variable! The standard deviation is a function of the individual sample size and varies with every sample.
P chart Interpretation - Same as for other control charts
P Chart - Examples
The clinic director's dilemma…
Is the scheduling problem due to cancellations or no shows?
The cancellations are in control, and therefore predictable. The no shows are not in control, a special cause occurred on 9/9.
