Quality Control
McGraw-Hill/Irwin
Copyright © 2012 by The McGraw-Hill Companies, Inc. All rights reserved.
 You should be able to:
1.
List and briefly explain the elements in the control process
2. Explain how control charts are used to monitor a process, and
the concepts that underlie their use
3. Use and interpret control charts
4. Perform run tests to check for nonrandomness in process output
5. Assess process capability
Student Slides
10-2
 Quality Control
 A process that evaluates output relative to a standard
and takes corrective action when output doesn’t meet
standards
 If results are acceptable no further action is required
 Unacceptable results call for correction action
Student Slides
10-3
 Inspection
 An appraisal activity that compares goods or services to
a standard
 Inspection issues:
1.
2.
3.
4.
Student Slides
How much to inspect and how often
At what points in the process to inspect
Whether to inspect in a centralized or on-site location
Whether to inspect attributes or variables
10-4
 Quality control seeks
 Quality of Conformance
 A product or service conforms to specifications
 A tool used to help in this process:
 SPC
 Statistical evaluation of the output of a process
 Helps us to decide if a process is “in control” or if corrective
action is needed
Student Slides
10-5
 Control Chart
 A time ordered plot of representative sample statistics
obtained from an ongoing process (e.g. sample means), used
to distinguish between random and nonrandom variability
 Control limits
 The dividing lines between random and nonrandom deviations
from the mean of the distribution
 Upper and lower control limits define the range of acceptable
variation
Student Slides
10-6
 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
Student Slides
10-7
k
k
x
 xi
i 1
R
R
i 1
k
where
x  Average of sample means
x i  mean of sample i
k  number of samples
Student Slides
i
k
where
R  Average of sample ranges
Ri  Range of sample i
10-8
 Used to monitor the central tendency of a process
x  chart Control Limits
UCLx  x  A2 R
LCLx  x  A2 R
where
A2  a control chart factor based on sample size, n
Student Slides
10-9
 Used to monitor process dispersion
R Chart Control Limits
UCLR  D4 R
LCLR  D3 R
where
D3  a control chart factor based on sample size, n
D4  a control chart factor based on sample size, n
Student Slides
10-10
Student Slides
10-11
 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
Student Slides
10-12
 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
 The inherent variability of process output (process width) relative
to the variation allowed by the design specification (specification
width)
Student Slides
10-13
Student Slides
10-14
 Quality is a primary consideration for nearly all
customers
 Achieving and maintaining quality standards is of
strategic importance to all business organizations
 Product and service design
 Increase capability in order to move from extensive use of
control charts and inspection to achieve desired quality
outcomes
Student Slides
10-15