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LECTURE 04C - STATISTICALLY-BASED QUALITY IMPROVEMENT FOR ATTRIBUTES Defects and Defectives; Charts:...

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LECTURE 04C - STATISTICALLY-BASED QUALITY IMPROVEMENT FOR ATTRIBUTES Defects and Defectives; Charts: p, np, c, u; Selection guide; SJSU Bus. 142 - David Bentley 1
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LECTURE 04C - STATISTICALLY-BASED QUALITY IMPROVEMENT FOR ATTRIBUTES

Defects and Defectives; Charts: p, np, c, u; Selection guide;

SJSU Bus. 142 - David Bentley 1

Topics

SJSU Bus. 142 - David Bentley 2

Generic process for developing attribute charts Understanding attribute charts

Generic process for developing attribute charts – (1)

SJSU Bus. 142 - David Bentley 3

1. Identify Critical Operations2. Identify critical product

characteristics3. Determine whether the product

characteristics is a variable or an attribute

Generic process for developing attribute charts – (2)

SJSU Bus. 142 - David Bentley 4

4. Select the appropriate process chart

5. Establish control limits6. Update the limits when changes

have been made to the process

UNDERSTANDING ATTRIBUTE CHARTS

SJSU Bus. 142 - David Bentley 5

Understanding Attribute Charts

SJSU Bus. 142 - David Bentley 6

Attribute Charts deal with binomial and Poisson processes, not measurements

(Remember: Variable charts deal with normal distributions!)

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

Defects

SJSU Bus. 142 - David Bentley 7

A defect is an irregularity or problem with a larger unit.

The larger unit may contain many defects

Defects are countable Example: bent pins on a fluorescent

bulb or pits in painted car door Defects are monitored using c

and u charts

Defectives

SJSU Bus. 142 - David Bentley 8

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

Example: fluorescent bulbs which don’t light or the car door

Defectives are monitored using p and np charts

CHARTS FOR DEFECTIVES

Defective units as a whole

SJSU Bus. 142 - David Bentley 9

Proportion or p Charts – (1)

SJSU Bus. 142 - David Bentley 10

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

Also known as fraction nonconforming chart or fraction defective chart or percentage chart

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

Proportion or p Charts – (2)

SJSU Bus. 142 - David Bentley 11

The p chart allows the sample size of each subgroup to be the same or to be different

p chart applications include: Late deliveries Incomplete orders Calls not getting dial tones Accounting transaction errors Parts that do not fit properly

SJSU Bus. 142 - David Bentley 12THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

SJSU Bus. 142 - David Bentley 13THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

Number or np Charts

SJSU Bus. 142 - David Bentley 14

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

Also known as number nonconforming chart

The np chart requires the sample size of each subgroup be the same

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

SJSU Bus. 142 - David Bentley 15THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

SJSU Bus. 142 - David Bentley 16THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

CHARTS FOR DEFECTSCountable non-conformances

SJSU Bus. 142 - David Bentley 17

c Charts

SJSU Bus. 142 - David Bentley 18

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

The units must be of the same sample space .. Size, height, length, volume …

Subgroup size is constant (the same) The c chart is used to detect

nonrandom events in the life of a production process

SJSU Bus. 142 - David Bentley 19THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

SJSU Bus. 142 - David Bentley 20THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

u Charts

SJSU Bus. 142 - David Bentley 21

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

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

Subgroup size is variable (different)

SJSU Bus. 142 - David Bentley 22THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

SJSU Bus. 142 - David Bentley 23THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

Summary of Charts for Attributes

• Fraction nonconforming (defective)• p-chart (proportion or percentage chart)• Fixed sample size, or• Variable sample size (units produced varies)

• np-chart for number nonconforming• Fraction x sample size (number in a sample)

• Charts for defects• c-chart (total number of defects/unit)• u-chart (average number of defects/unit)

SJSU Bus. 142 - David Bentley 24THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM (Mod. 10/14/02 DAB)

Control Chart Formulas

SJSU Bus. 142 - David Bentley 25

Control Chart Selection

SJSU Bus. 142 - David Bentley 26

Quality Characteristicvariable attribute

n>1?

n>=10 or computer?

x and MRno

yes

x and s

x and Rno

yes

defective defect

constant sample size?

p-chart withvariable samplesize

no

p ornp

yes constantsampling unit?

c u

yes no

THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson LearningTM

Summary

SJSU Bus. 142 - David Bentley 27

The object of using process charts is to continually improve your processes

As we make changes and improvements… there will be fewer defects and

defectives


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