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

5. Cpk = Minimum of
Upper specification – x
3
x – Lower specification
3 ,
= =
Process Capability Index, Cpk, is an index that measures the potential
for a process to generate defective outputs relative to either upper
or lower specifications.
Process Capability Index, Cpk
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. Cpk = Minimum of
Upper specification – x
3
x – Lower specification
3 ,
= =
Upper specification = 30 minutes
Lower specification = 20 minutes
Average service = 26.2 minutes
 = 1.35 minutes
Intensive Care Lab
Assessing Process Capability
Cpk = Minimum of
26.2 – 20.0
3(1.35)
,
30.0 – 26.2
3(1.35)
Cpk = Minimum of 1.53, 0.94 = 0.94
Process
Capability
Index
Example

8. Cp =
Upper specification - Lower specification
6
Cp =
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 Cpk = 0.94 Cp = 1.23
Intensive Care Lab
Assessing Process Capability
Does not meet 3 (1.00 Cpk) target due to a shift in mean
(Note variability is ok since Cp is over 1.0)
Example