Operations management by stevenson 9th student slides chapter 10

Chapter 10Quality Control... Statistical Process Control SPC• Quality control seeks – Quality of Conformance • A product or service conforms to specifications • A tool used to help in th

Chapter 10

Quality Control

Chapter 10: Learning Objectives

• You should be able to:

Statistical Process Control (SPC)

• Quality control seeks

– Quality of Conformance

• A product or service conforms to specifications

• A tool used to help in this process:

• Statistical evaluation of the output of a process

• Helps us to decide if a process is “in control” or if corrective

action is needed

Process Variability

• All processes generate output that exhibits

some degree of variability

• The fundamental question:

– Is the variation within acceptable boundaries?

Control Chart

UCL

LCL

Mean

Out of control

Normal

variation

due to

chance

Abnormal variation

Abnormal variation due to assignable causes

Each point on the control chart represents a sample of n

observations

Control Charts for Variables

• Variables generate data that are measured

– Mean control charts

• Used to monitor the central tendency of a process.

– X bar charts

– Range control charts

• Used to monitor the process dispersion

– R charts

Establishing Control Limits

samples of

number

sample of

mean

means sample

of Average

where

1













k

i x

x

k

x x

i

k

i

i

i R

k R

i

k i

i

sample of

Range

ranges sample

of Average R

where











X-Bar Chart: Control Limits

• Used to monitor the central tendency of a

process

R A x

LCL

R A x

UCL

x x

where

Limits Control

chart x

2

2











Range Chart: Control Limits

• Used to monitor process dispersion

n D

n D

R D LCL

R D UCL

R R

size, sample

on based

factor chart

control a

size, sample

on based

factor chart

control a

where

Limits Control

Chart R

4 3

3 4









Control Charts for Attributes

• Attributes generate data that are counted.

– p-Chart

• Control chart used to monitor the proportion of

defectives in a process

– c-Chart

• Control chart used to monitor the number of defects per

unit

Process Capability

• Once a process has been determined to be stable, it is

necessary to determine if the process is capable of producing

output that is within an acceptable range

– Tolerances or specifications

• Range of acceptable values established by engineering design or customer requirements

– Process variability

• Natural or inherent variability in a process – Process capability

• Process variability relative to specification

Cp : Process Capability Ratio

limit ification)

rance(spec lower tole

LTL

limit tion)

(specifica rance

upper tole UTL

where

6

LTL

-UTL









p

C

Cpk : Process Capability Index

• Used when a process is not centered at its

target, or nominal, value

















LTL ,

3

UTL min

,

min

x x

C C

Taguchi Loss Function

Operations Strategy

• Quality is a primary consideration for nearly all

customers

– Achieving and maintaining quality standards is of

strategic importance to all business organizations

of control charts and inspection to achieve desired quality outcomes