Quality Management and Location Planning (critical thinking 9)
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SEU_MGT530_Module09_PPT_Ch10.pptxQuality Control
Chapter 10
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You should be able to:
LO 10.1 Explain the need for quality control
LO 10.2 Discuss the basic issues of inspection
LO 10.3 List and briefly explain the elements of the control process
LO 10.4 Explain how control charts are used to monitor a process, and the concepts that underlie their use
LO 10.5 Use and interpret control charts
LO 10.6 Perform run tests to check for nonrandomness in process output
LO 10.7 Assess process capability
Chapter 10: Learning Objectives
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What is Quality Control?
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
Inspection alone is not is generally not sufficient to achieve a reasonable level of quality
Most organization rely upon some inspection and a great deal of process control to achieve an acceptable level of quality
LO 10.1
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Phases of Quality Assurance
LO 10.1
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Inspection
An appraisal activity that compares goods or services to a standard
Inspection issues:
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
Inspection
LO 10.2
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How Much to Inspect
LO 10.2
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Where to Inspect in the Process
Typical inspection points:
Raw materials and purchased parts
Finished products
Before a costly operation
Before an irreversible process
Before a covering process
LO 10.2
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Effects on cost and level of disruption are a major issue in selecting centralized vs. on-site inspection
Centralized
Specialized tests that may best be completed in a lab
More specialized testing equipment
More favorable testing environment
On-Site
Quicker decisions are rendered
Avoid introduction of extraneous factors
Quality at the source
Centralized vs. On-Site Inspection
LO 10.2
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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
Statistical Process Control (SPC)
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Two basic questions: concerning variability:
Issue of process control
Are the variations random? If nonrandom variation is present, the process is said to be unstable.
Issue of process capability
Given a stable process, is the inherent variability of the process within a range that conforms to performance criteria?
Process Variability
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Variation
Variation
Random (common cause) variation:
Natural variation in the output of a process, created by countless minor factors
Assignable (special cause) variation:
A variation whose cause can be identified
A nonrandom variation
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Sampling and Sampling Distribution
SPC involves periodically taking samples of process output and computing sample statistics:
Sample means
The number of occurrences of some outcome
Sample statistics are used to judge the randomness of process variation
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Sampling and corrective action are only a part of the control process
Steps required for effective control:
Define: What is to be controlled?
Measure: How will measurement be accomplished?
Compare: There must be a standard of comparison
Evaluate: Establish a definition of out of control
Correct: Uncover the cause of nonrandom variability and fix it
Monitor: Verify that the problem has been eliminated
Control Process
LO 10.3
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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
Control Charts: The Voice of the Process
LO 10.4
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Control Chart
Each point on the control chart represents a sample of n observations
LO 10.4
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Type I error
Concluding a process is not in control when it actually is.
The probability of rejecting the null hypothesis when the null hypothesis is true.
Manufacturer’s risk
Type II error
Concluding a process is in control when it is not.
The probability of failing to reject the null hypothesis when the null hypothesis is false.
Consumer’s risk
Errors
LO 10.4
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Type I Error
LO 10.4
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Observations from Sample Distribution
LO 10.4
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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
LO 10.5
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Establishing Control Limits
LO 10.5
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Used to monitor the central tendency of a process
X-Bar Chart: Control Limits
LO 10.5
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Used to monitor process dispersion
Range Chart: Control Limits
LO 10.5
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Factors for Three Sigma Control Charts
LO 10.5
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Mean and Range Charts
LO 10.5
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To determine initial control limits:
Obtain 20 to 25 samples
Compute appropriate sample statistics
Establish preliminary control limits
Determine if any points fall outside of the control limits
If you find no out-of-control signals, assume the process is in control
If you find an out-of-control signal, search for and correct the assignable cause of variation
Resume the process and collect another set of observations on which to base control limits
Plot the data on the control chart and check for out-of-control signals
Using Mean and Range Charts
LO 10.5
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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
Control Charts for Attributes
LO 10.5
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When observations can be placed into two categories
Good or bad
Pass or fail
Operate or don’t operate
When the data consists of multiple samples of several observations each
Use a p-chart:
LO 10.5
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p-chart Control Limits
LO 10.5
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Use only when the number of occurrences per unit of measure can be counted; non-occurrences cannot be counted.
Scratches, chips, dents, or errors per item
Cracks or faults per unit of distance
Breaks or tears per unit of area
Bacteria or pollutants per unit of volume
Calls, complaints, failures per unit of time
Use a c-chart:
LO 10.5
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At what points in the process to use control charts
What size samples to take
Sample frequency
What type of control chart to use
Variables
Attributes
Managerial Considerations
LO 10.5
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Run Tests
Even if a process appears to be in control, the data may still not reflect a random process
Analysts often supplement control charts with a run test
Run test
A test for patterns in a sequence
Run
Sequence of observations with a certain characteristic
LO 10.6
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Nonrandom Patterns
LO 10.6
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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)
Process Capability
LO 10.7
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Process Capability (cont.)
Lower Specification
Upper Specification
Process variability (width) is less than the specification width
Lower Specification
Upper Specification
Process variability (width) matches specifications width
Lower Specification
Upper Specification
Process variability (width) exceeds specifications
LO 10.7
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Cp
LO 10.7
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Used when a process is not centered at its target, or nominal, value
Cpk
LO 10.7
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Improving Process Capability
Simplify
Standardize
Mistake-proof
Upgrade equipment
Automate
LO 10.7
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Taguchi Loss Function
LO 10.7
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There are several risks of using capability measures:
The process may not be stable
The process output may not be normally distributed
The process not centered but Cp is used
Limitations of Capability Measures
LO 10.7
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Operations Strategy
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
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