LECTURE 04C - STATISTICALLY-BASED QUALITY IMPROVEMENT FOR ATTRIBUTES

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

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Topics

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Generic process for developing attribute charts Understanding attribute charts

Generic process for developing attribute charts – (1)

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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)

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4. Select the appropriate process chart

5. Establish control limits6. Update the limits when changes

have been made to the process

UNDERSTANDING ATTRIBUTE CHARTS

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Understanding Attribute Charts

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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

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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

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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

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Proportion or p Charts – (1)

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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)

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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

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Number or np Charts

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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

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CHARTS FOR DEFECTSCountable non-conformances

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c Charts

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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

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u Charts

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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)

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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)

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Control Chart Formulas

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Control Chart Selection

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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

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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