Use when the mean cannot be easily adjusted ie cycle

Use when the mean cannot be easily adjusted (i.e., Cycle
times, customer satisfaction indices, etc.)
Typical goals for Cpk are greater than 1.33 (or 1.67 if
safety related)
For Cpk Std. Deviation estimates use:
Rbar/d
2
[short term] (calculated from Xbar-R chart)
Released: 20160801 Process Capability
BB032.10
A Metaphor for C
p
– Parking Vehicles 1
Released: 20160801 Process Capability
C
p
§ 1.3
+/ - 3 ı
(Voice of Process)
+/ - 3 ı
(Voice of Process)
+/ - 3 ı
(Voice of Process)
Voice of Customer
For Cp, it doesn’t matter where the process is relative to the specifications,
only the width of the process to the width of the specifications.
268

BB032.11
A Metaphor for C
pk
– Parking Vehicles 2
Released: 20160801 Process Capability
C
pk
§ 1.3
C
pk
§ 1.3: the process has room to move before exceeding a
customer specification. In other words, at least the driver and/or
the passenger can get out of the HMMWV .
+/ - 3 ı
(Voice of Process)
BB032.12
A Metaphor for C
pk
– Parking Vehicles 2
Released: 20160801 Process Capability
C
pk
§ 0
Specifications
(Voice of Customer)
&SN§±² The center of the process is on (or equal to) a customer
specification (either side)!
+/ - 3 ı
(Voice of Process)
269

BB032.13
A Metaphor for C
pk
– Parking Vehicles 2
Released: 20160801 Process Capability
C
pk
§ -1
&SN§ -1: The center of the process is outside the customer specifications
(either side)!
Specifications
(Voice of Customer)
+/ - 3 ı
(Voice of Process)
BB032.14
Uses of Capability Analysis
Performed on existing processes as a means of
establishing a baseline of current operations (so it’s
possible to tell when improvement has occurred)
When done periodically, is a means of monitoring
deterioration of a process for whatever reason
(system, personnel, environment, etc.)
Can be done on any process that has a target spec.
established (target spec. is needed for the values in
numerator), and has a capable measuring system
(needed for valid values in denominator)
Released: 20160801 Process Capability
270

BB032.15
Process Capability – CPU
CPU indicates capability against an Upper
Specification Limit. In the next example, the average
delivery time is within the 30 minute requirement.
However, the histogram shows that quite a few
deliveries are exceeding the 30 minute upper spec.
limit
The CPU figure of 0.139 confirms that the process is
incapable (<1)
Released: 20160801 Process Capability
BB032.16
Process Capability – CPU
Released: 20160801 Process Capability
USL- X
C
pu
=
3s
271

BB032.17
Released: 20160801 Process Capability
Minitab Exercise
Open Minitab file: Message Delivery
Click S tat>Q uality Tools>Capability S ixpack>Normal
Note: This exercise is based on
a different dataset than previous
slides so different, unrelated
results can be expected.
BB032.18
Released: 20160801 Process Capability
Minitab Exercise (Cont.)
1. Double click on
C-5 Delivery Time to
Place it in the box for
Single column
2. In the Subgroup size
Box, type a 5 since we
took 5 data points per day
4. Click on OK
3. For Upper Spec type
in 30 minutes (given)
– we have no lower Spec – so leave BLANK!