basics of capability-2

8/11/2019 Basics of Capability-2

1/19

8-1

IsProcessCapable

?TheQuality

ImprovementModel

Use SPC toMaintainCurrentProcess

Collect &Interpret

Data

SelectMeasures

DefineProcess

Is

ProcessCapable?

ImproveProcess

Capability

IsProcessStable

?

Investigate &Fix Special

Causes

No

Yes

No

Yes

Is Process Capable?

Purpose: Determine the adequacyof the process withrespect to customer

/management needs.


2/19

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.


3/19

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


4/19

8-4

IsProcessCapable

?

Capability Assessment for Counting Measures

Order Entry Process

Is this process adequate as is?

Should it be improved?

Week

123456

78910111213

14151617181920

Total

# Errors

15221810139

2712242288

26

162010169

1520

320

0

5

10

15

20

25

30

35

5 10 15 20

Avg=16.0

LCL=4.0

UCL=28.0

Week

Numberof Errors

Control Chart


5/19

8-5

IsProcessCapable

?

Measures of Process CapabilityCp=

Specification RangeTrue Process Range

= USL - LSL6s

c

Cpk =

Distance from process averageto closest specification limit

12True Process Range

= min (USL - x, x- LSL)

3sc

Problem: We are assuming the process has a target that isin the center of the specification range, and thatthe 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 exactlybetween 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 specs

Cpk = 1.0 Process is marginally capableCpk > 1.0 Process is capable of meeting specs

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


6/19

8-6

IsProcessCapable

?

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

In the long term?

USLLSL

Process Capability


7/198-7

IsProcessCapable

?Process Capability Ratios

Voice of The Customer

Voice of The Process

USLLSL


8/198-8

IsProcessCapable

?


9/198-9

IsProcessCapable

?Process Capability - The Strategy

CenteringThe Process Is On Target

SpreadReduce The Variation

LSL USL

DefectsDefects


10/198-10

IsProcessCapable

?

2 Key Metrics for Measuring Capability

C Min(X - LSL

3

USL- X

3pk , )

c

LSLUSLCp

s6

cs cs

Process Capability Ratios


11/198-11

IsProcessCapable

?Process Capability Ratios - Concept

Total ToleranceC

Process Spreadp

c

LSLUSLCp

s6


12/198-12

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 Process

In Control ?

Is It Producing

Defects ?

35302520151050

39

34

29

24

Observation Number

In

dividualValue

X=30.60

3.0SL=37.36

-3.0SL=23.84

CP& CPKMeasure Short-termCapability


13/198-13

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 Process

In Control ?

Is It Producing

Defects ?

100500

50

40

30

20

Observation Number

I

ndividualValue

X=33.80

3.0SL=47.12

-3.0SL=20.49

Long Term Performance

Short termCapability


14/19

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 thelong-term data?

What implication does this have in doing capability

studies?

Descriptive Statistics

Variable N Mean StdDevshort term 30 30.6 2.23

long term 180 33.8 4.44


15/19

8-15

IsProcessCapable

?

Measures of Process PerformancePp=

Specification RangeTrue Process Range

= USL - LSL6ss

Ppk =

Distance from process averageto closest specification limit

12

True Process Range=

min (USL - x, x- LSL)3ss

Problem: We are assuming the process has a target that isin the center of the specification range, and thatthe 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

Pp< 1.0 Process Performance is not meeting specs

Pp= 1.0 Process Performance is marginally meeting specs

Pp> 1.0 Process Performance is meeting specs

Ppk< 1.0 Process Performance is not meeting specs

Ppk= 1.0 Process Performance is marginally meeting specs

Ppk> 1.0 Process Performance is meeting specs


16/19

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

C

O2-Shrt

CO2 Levels for 55 Time PointsDays Sales Outstanding for 55 Days

DSO


17/19

8-17

IsProcessCapable

?Capability vs. Performance

5040302010

14

13

12

11

10

Index

CO2-Shrt

CO2 Levels for 55 Time PointsDays Sales Outstanding for 55 Days

D

SO

Process Performance:

Total Variation including

shifts and drifts

(Pp & Ppk)

Capability: Only

random or short

term variability

(Cp & Cpk)


18/19

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

3

USL- X

3pk , )

s

LSLUSLpP

s6

ss ss


19/19

8-19

IsProcessCapable

?

WARNING!!!

Statistical Assumptions Made In Capability Studies

1. Data Comes From a Stable Process

If not, work towards getting the process incontrol

Dont despair, you can still make someassumptions about your process in the meantime

basics of capability-2

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