Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall. 12- 1 Chapter 12 Statistically-Based Quality Improvement for Attributes

Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall.12- 1

Chapter 12

Statistically-Based Quality Improvement

for Attributes


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Chapter 12

Types of attributesGeneric process for developing attribute chartsUnderstanding attribute chartsChoosing the right attributes chartReliability models


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Types of Attributes

Types of Attributes

StructuralSensoryPerformanceTemporalEthical


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Generic process for developing attribute charts

1. Identify Critical Operations

2. Identify critical product characteristics

3. Determine whether the product characteristics is a variable or an attribute


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Generic process for developing attribute charts

4. Select the appropriate process chart

5. Establish control limits

6. Update the limits when changes have been made to the process


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Understanding Attribute Charts

Attribute Charts deal with binomial and Poisson processes, not measurements

Think in terms of defects and defectives rather than diameters and widths



A defect is an irregularity or problem with a larger unit

The larger unit may contain many defects Defects are countable



A defective is a unit that, as a whole, is not acceptable or does not meet performance requirements

Defectives are monitored p and np charts

Defects are monitored using c and u charts



A p chart is a process chart that is used to graph the proportion of items in a sample that are defective

Effective to determine when there has been a shift in the proportion defectives for a particular product or service



p chart applications include: Late deliveries Incomplete orders Calls not getting dial tones Accounting transaction errors Clerical errors on written forms Parts that do not mate properly



p charts for example 12-1



Example 12-1 using excel



The np chart is a graph of the number of defectives in a subgroup

The np chart requires the sample size of each subgroup be the same each time a sample is drawn

The uses for the np chart are essentially the same as the p chart



np charts for example 12-2



Example 12-2 using excel



The c chart is a graph of the number of defects per unit

The units must be of the same metric .. size, height, length, volume …

The c chart is used to detect nonrandom events in the life of a production process



c chart



The u chart is a graph of the average number of defects per unit.

The units can be of different Sizes, height, length, volume …



u chart



c and u charts using excel


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Choosing the Right Attributes Chart


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Reliability Models

Bathtub shaped function The vertical axis is the failure rate The horizontal axis is time Shows products are

more likely to fail either

early or late in their

useful lifetimes



Series reliability Components are in a series if the

performance of the entire system depends on all the components functioning properly

The components need not be physically wired sequentially

All parts must function for the system to function



Parallel reliability High reliability systems often require

extremely require high component reliability

When it is impossible an alternative is to use a backup system

Another word for backup is redundant or parallel



Measuring reliability

Failure rate = l = number of failures>(units tested * number of hours tested)

Mean Time to failure =

Mean Time Between Failures = total operating hours>number of failures



System Availability

System availability = MTBF>(MTBF + MTTR)


Strategic Quality PlanningStatistically-Based Quality Improvement for Attributes Summary

Types of attributesGeneric process for developing attribute chartsUnderstanding attribute chartsChoosing the right attributes chartReliability models


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Copyright © 2013 Pearson Education, Inc. Publishing as Prentice Hall. 12- 1 Chapter 12 Statistically-Based Quality Improvement for Attributes