8-1

IsProcessCapable

?TheQualityImprovementModel

Use SPC to Maintain Current Process

Collect & Interpret

Data

Select Measures

Define Process

IsProcessCapable

?

Improve Process

Capability

IsProcessStable

?

Investigate & Fix

Special Causes

No

Yes

No

Yes

Is Process Capable?

Purpose:Determine the adequacy of the process with respect to customer /management needs.

8-2

IsProcessCapable

?Capable Process

A stable process that meets customer requirements.

8 22 242 4 6 10 12 14 16 18 20 26 28 30 32

Run Order

UCL

CL

LCL

Control Chart

Lower Spec Upper SpecTarget

Histogram

Capability assessments for unstable processes, may not be indicative of how the process is actually performing.

8-3

IsProcessCapable

?

Assessing Process Capability

Counting Measures The average percent defectives. The average number of defects.

Instrument Measures Comparing both the center of the process and the

process variation

8-4

IsProcessCapable

?

Capability Assessment for Counting Measures

“Order Entry Process”

Is this process adequate as is?Should it be improved?

Week123456789

1011121314151617181920

Total

# Errors15221810139

2712242288

26162010169

1520

320

05

101520253035

5 10 15 20

Avg=16.0

LCL=4.0

UCL=28.0

Week

Numberof Errors

Control Chart

8-5

IsProcessCapable

?

Measures of Process CapabilityCp = Specification Range

True Process Range = USL - LSL

6c

Cpk = Distance from process averageto closest specification limit

12 True Process Range

= min (USL - x , x - LSL)3c

Problem: We are assuming the process has a target that is in the center of the specification range, and that the process is in fact centered on that target.

Note: a negative result is possible if the process average is outside specifications

Benefits: • Optimal values are attained by running exactly between specs. • Can (must) be used for 1-sided specifications

Cp < 1.0 Process is not capable of meeting specsCp = 1.0 Process is marginally capableCp > 1.0 Process is capable of meeting specs

Cpk < 1.0 Process is not capable of meeting specsCpk = 1.0 Process is marginally capableCpk > 1.0 Process is capable of meeting specs

Warning: Capability assessments for unstable processes, may not be indicative of how the process is actually performing.

8-6

IsProcessCapable

?

How much material is out of spec? In the short term? In the long term?

USLLSL

Process Capability

8-7

IsProcessCapable

?Process Capability Ratios

Voice of The Customer

Voice of The Process

USLLSL

8-8

IsProcessCapable

?

8-9

IsProcessCapable

?Process Capability - The Strategy

Centering –The Process Is On Target

Spread – Reduce The Variation

LSL USL

DefectsDefects

8-10

IsProcessCapable

?

2 Key Metrics for Measuring Capability

C Min( X-LSL

3USL- X

3pk

, )

cLSLUSLCp

6

c c

Process Capability Ratios

8-11

IsProcessCapable

?Process Capability Ratios - Concept

Total ToleranceC

Process Spreadp

cLSLUSLCp

6

8-12

IsProcessCapable

?

A Short-term Capability study covers a relatively short period of time (days, weeks) generally consisting of 30 to 50 data points. The actual number depends on the subject under study.

Is The ProcessIn Control ?

Is It Producing Defects ?

35302520151050

39

34

29

24

Observation Number

Indi

vidu

al V

alue

I Chart for C1

X=30.60

3.0SL=37.36

-3.0SL=23.84

CP & CPK Measure Short-term Capability

8-13

IsProcessCapable

?

A long-term capability study covers a relatively long period of time (weeks, months) generally consisting of 100-200 data points. Again, the actual amount depends

on the subject under study.

Is The ProcessIn Control ?

Is It Producing Defects ?

100500

50

40

30

20

Observation Number

Indi

vidu

al V

alue

I Chart for C3

X=33.80

3.0SL=47.12

-3.0SL=20.49

Long Term Performance

Short term Capability

8-14

IsProcessCapable

?

A Further Look at Capability

Compare the estimates of the process deviations from the short-term and long-term data

What is the difference between the short-term and the long-term data?

What implication does this have in doing capability studies?

Descriptive Statistics

Variable N Mean StdDev short term 30 30.6 2.23 long term 180 33.8 4.44

8-15

IsProcessCapable

?

Measures of Process PerformanceP p = Specification Range

True Process Range = USL - LSL

6s

Ppk =

Distance from process averageto closest specification limit

12 True Process Range

= min (USL - x , x - LSL)3s

Problem: We are assuming the process has a target that is in the center of the specification range, and that the process is in fact centered on that target.

Note: a negative result is possible if the process average is outside specifications

Benefits: • Optimal values are attained by running exactly between specs. • Can (must) be used for 1-sided specifications

P p < 1.0 Process Performance is not meeting specsP p = 1.0 Process Performance is marginally meeting specsP p > 1.0 Process Performance is meeting specs

Ppk < 1.0 Process Performance is not meeting specsPpk = 1.0 Process Performance is marginally meeting specsPpk > 1.0 Process Performance is meeting specs

8-16

IsProcessCapable

?

Performance vs. Capability

These data show that the process, if well controlled can perform much better than it currently is

5040302010

14

13

12

11

10

Index

CO

2-S

hrt

CO2 Levels for 55 Time PointsDays Sales Outstanding for 55 Days

DS

O

8-17

IsProcessCapable

?Capability vs. Performance

5040302010

14

13

12

11

10

Index

CO

2-S

hrt

CO2 Levels for 55 Time PointsDays Sales Outstanding for 55 Days

DS

O

Process Performance: Total Variation including

shifts and drifts

(Pp & Ppk)

Capability: Only random or short term variability

(Cp & Cpk)

8-18

IsProcessCapable

?

Process Performance Ratios

The P-family of indices are computationally the same as the C-family of ‘capability’ indices, but use the observed

long-term standard deviation.

P Min( X-LSL

3USL- X

3pk

, )

sLSLUSL

pP6

s s

8-19

IsProcessCapable

?WARNING!!!WARNING!!!

Statistical Assumptions Made In Capability Studies

1. Data Comes From a Stable ProcessData Comes From a Stable Process If not, work towards getting the process in

control Don’t despair, you can still make some

assumptions about your process in the mean time