Lean six sigma process improvement tools and techniques by donna summers chapter 19

They need the answer to the question: How do we know the process will generate products or services that meet the customer’s specifications?... Process Capability• Process capability ref

Process Capability

Chapter 19

Process Capability

• She had the virtuous sense of one who has made an

intuitive leap from the most fragile collection of evidence.

• Lean Six Sigma practitioners know that they can not

make quick judgments without the data to support their

conclusions They need evidence They need the

answer to the question: How do we know the process will generate products or services that meet the customer’s

specifications?

Process Capability

• Process capability refers to the

ability of a process to produce

products or provide services capable

of meeting the specifications set by

the customer or designer.

Process Capability

• The relationship between the population

standard deviation  and the standard

deviation of the subgroup averages is

shown by the formula:

 X  

n

Process Capability

• The population standard deviation can be

estimated from either the standard

deviation associated with the sample

standard deviation (s) or the range (R):

ö

c 4 or  ö 

R

d 2

Process Capability

• CONTROL LIMITS VERSUS SPECIFICATION LIMITS

– Specifications communicate what the customers

expect, want, or need from the process

Specifications can be considered the voice of the

customer

– Control limits are the voice of the process The

centerline on the X-bar chart represents process

centering The R and s chart limits represent the

amount of variation present in the process Control

limits are a prediction of the variation that the process will exhibit in the near future

Process Capability

Case I

– The 6 spread of the process individuals is shown by

the solid line As expected, the spread of the

individual values is greater than the spread of the

averages; however, the values are still within the

specification limits The 6 spread of the individuals is less than the spread of the specifications This allows

for more room for process shifts while staying within

the specifications Notice that even if the process

drifts out of control, the change must be dramatic

before the parts are considered out of specification

6  USL  LSL

Process Capability

Case II

• In this situation, 6  is equal to the tolerance

As long as the process remains in control

and centered, with no change in process

variation, the parts produced will be within

specification However, a shift in the process

mean will result in the production of parts that are out of specification An increase in the

variation present in the process also creates

an out-of-specification situation.

6 USL  LSL

Process Capability

Case III

• Any time that the 6  spread is greater than the tolerance spread, an undesirable

situation exists Even though the process

is exhibiting only natural patterns of

variation, it is incapable of meeting the

specifications set by the customer.

6  USL  LSL

Process Capability

• CALCULATING PROCESS CAPABILITY

INDICES

• Capability Index Cp

6 ö 

Process Capability

• CALCULATING PROCESS CAPABILITY

INDICES

• Capability Ratio Cr

C r  6 ö 

USL  LSL

Process Capability

• CALCULATING PROCESS CAPABILITY

INDICES

3

ö

ö



Process Capability

INTERPRETING PROCESS CAPABILITY INDICES

1 When Cp has a value of 1.0 or greater, the process is producing

product capable of meeting specifications.

2 The Cp value does not reflect process centering.

3 When the process is centered, Cp = Cpk

4 Cpk s always less than or equal to Cp

5 When Cpis greater than or equal to 1.0 and Cpk has a value of 1.00

or more, it indicates the process is producing product that conforms

to specifications.

6 When Cpk has a value less than 1.00, it indicates the process is

producing product that does not conform to specifications.

7 A Cp value of less than 1.00 indicates that the process is not

capable.

8 A Cpk value of zero indicates the process average is equal to one of the specification limits.