Process capability

Process capability

1. Process Capability Process capability is the ability of the process to meet the design specifications for a service or product. Nominal value is a target for design specifications. Tolerance is an allowance above or below the nominal value.

2. 20 25 30 Upper specification Lower specification Nominal value Process Capability Process is capable Process distribution

3. Process is not capable 20 25 30 Upper specification Lower specification Nominal value Process distribution Process Capability

4. Process capability ratio, C p , is the tolerance width divided by 6 standard deviations (process variability). Process Capability Ratio, C p C p = Upper specification - Lower specification 6 

5. C pk = Minimum of Upper specification – x 3 s x – Lower specification 3 s , = = Process Capability Index, C pk , is an index that measures the potential for a process to generate defective outputs relative to either upper or lower specifications. Process Capability Index, C pk We take the minimum of the two ratios because it gives the worst-case situation.

6. Intensive Care Lab Example Upper specification = 30 minutes Lower specification = 20 minutes Average service = 26.2 minutes  = 1.35 minutes The intensive care unit lab process has an average turnaround time of 26.2 minutes and a standard deviation of 1.35 minutes. The nominal value for this service is 25 minutes with an upper specification limit of 30 minutes and a lower specification limit of 20 minutes. The administrator of the lab wants to have three-sigma performance for her lab. Is the lab process capable of this level of performance?

7. C pk = Minimum of Upper specification – x 3 s x – Lower specification 3 s , = = Upper specification = 30 minutes Lower specification = 20 minutes Average service = 26.2 minutes  = 1.35 minutes Intensive Care Lab Assessing Process Capability C pk = Minimum of 26.2 – 20.0 3( 1.35 ) , 30.0 – 26.2 3( 1.35 ) C pk = Minimum of 1.53, 0.94 = 0.94 Process Capability Index Example

8. C p = Upper specification - Lower specification 6  C p = 30 - 20 6(1.35) = 1.23 Process Capability Ratio Before Process Modification Upper specification = 30.0 minutes Lower specification = 20.0 minutes Average service = 26.2 minutes  = 1.35 minutes C pk = 0.94 C p = 1.23 Intensive Care Lab Assessing Process Capability Does not meet 3  (1.00 C pk ) target due to a shift in mean (Note variability is ok since C p is over 1.0) Example