19. Process Capability

Question 1

Process Capability P.365

Answer 1

Inherent variability of the characteristics of a process. A process is capable when the output always conforms to the process specifications (6σ).

Question 2

Process Capability Index P.365

Answer 2

Used to represent the ability to meet customer specification limits..

Question 3

Cp Index

Answer 3

Describes process capability in relation to the specified tolerance of a characteristic divided by the natural process variation

Cp = USL - LSL / 6σ 
1 = capable
2= represent 6σ performance

Question 4

Cpk Index

Answer 4

Cpk = min (Cpkl, Cpku)

Cpkl = x̅- LSL / 3σ
Cpku = USL - x̅ / 3σ

1. 33 = goals
2. 67 = mission critical / safety items

Question 5

Cpm Index

Answer 5

Process capability index of the mean. An index that accounts for the location of the process average relative to a target value.

Question 6

Process performance P.370

Answer 6

Statistical measure of the outcome of a characteristic from a process that may not have been demonstrated to be in a state of a statistical control.

Question 7

Process Performance Index P.371

Answer 7

A dimensionless number that is used to represent the ability to meet specification limits on a characteristic of interest.

Tend to be a bit larger than their corresponding process capability indices since they are often based on smaller sample sizes.

Question 8

Pp Index

Answer 8

Pp = USL - LSL / 6s

Question 9

Ppk Index

Answer 9

Ppk = min. (Ppkl, Ppku)

Question 10

Ppm Index

Answer 10

Analogoous to Cpm

Question 11

Conducting a Process Capability Study P.372

Answer 11

1. Select a quality characteristic of a specific process for study.
2. Confirm the measurement system.
3. Gather the data.
4. Verify process stability and ensure that the process is in control.
5. Verify that the individual data are normally distributed.
6. Obtain the process specifications.
7. Determine process capability indices and interpret them.
8. Update the process control plan.

Question 12

Lower confidence Limit on the Cpk P.373

Answer 12

LCL Cpk = Cpk - Zα√(1/9n+(Cpk^2)/(2n-2))

Question 13

Process Capability for Attributes Data P.374

Answer 13

Equivalent Ppk% = Equivalent Ppk based on percent

Equivalent Pp% = Equivalant Pp based on percent

Question 14

Process Capability for Non-Normal Data P.377

Answer 14

• Find another known distribution that fits the data.
• Use nonlinear regression to fit a curve to the data.
• Transform the date to produce a second variable that is normally distributed.
• Box-Cox
• Johnson transformation
• Weibull

Question 15

Natural Process Limit P.381

Answer 15

+- 3σ limits (6σ spread) around a process averages. (VoP)

Question 16

Specification Limits P.381

Answer 16

Customer determined (VoC)

Question 17

Process Performance Metrics P.381

Answer 17

• Percentage Defective
• PPM
• Defects per Unit (DPU)
• Defects per Million Opportunities (DPMO)
• Throughput Yield (TPY), first time yield
• Rolled Throughput Yield (RTY)

Question 18

Short-Term Capability P. 388

Answer 18

• Cp, Cpk, Cpm
• Process must be in statistical control
• These indices are derived using an estimate of short-term variation.
• It is assumed approximately normal distribution
• Specification limits are based on customer requirements

Question 19

Long-Term Capability

Answer 19

• Pp, Ppk, Ppm
• Process does not need be in statistical control
• These indices are derived using an estimate of long-term variation.
• It is assumed approximately normal distribution
• Specification limits are based on customer requirements

Question 20

Relationship between Long/ Short term Capability P.389

Answer 20

Long-term variability = Short-term variability +- 1.5σ shift.