LECTURE 04C - STATISTICALLY-BASED QUALITY IMPROVEMENT FOR ATTRIBUTES Defects and Defectives; Charts: p, np, c, u; Selection guide; SJSU Bus David Bentley1

Topics SJSU Bus David Bentley2  Generic process for developing attribute charts  Understanding attribute charts

Generic process for developing attribute charts – (1) SJSU Bus David Bentley3 1. Identify Critical Operations 2. Identify critical product characteristics 3. Determine whether the product characteristics is a variable or an attribute

Generic process for developing attribute charts – (2) SJSU Bus David Bentley4 4. Select the appropriate process chart 5. Establish control limits 6. Update the limits when changes have been made to the process

UNDERSTANDING ATTRIBUTE CHARTS SJSU Bus David Bentley5

Understanding Attribute Charts SJSU Bus David Bentley6  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 David Bentley7  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 David Bentley8  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 David Bentley9

Proportion or p Charts – (1) SJSU Bus David Bentley10  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 David Bentley11  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 David Bentley12 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

SJSU Bus David Bentley13 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

Number or np Charts SJSU Bus David Bentley14  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 David Bentley15 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

SJSU Bus David Bentley16 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

CHARTS FOR DEFECTS Countable non-conformances SJSU Bus David Bentley17

c Charts SJSU Bus David Bentley18  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 David Bentley19 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

SJSU Bus David Bentley20 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

u Charts SJSU Bus David Bentley21  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 David Bentley22 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

SJSU Bus David Bentley23 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

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 David Bentley24 THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM (Mod. 10/14/02 DAB)

Control Chart Formulas SJSU Bus David Bentley25

Control Chart Selection SJSU Bus David Bentley26 Quality Characteristic variableattribute n>1? n>=10 or computer? x and MR no yes x and s x and R no yes defectivedefect constant sample size? p-chart with variable sample size no p or np yes constant sampling unit? c u yesno THE MANAGEMENT AND CONTROL OF QUALITY, 5e, © 2002 South-Western/Thomson Learning TM

Summary SJSU Bus David Bentley27  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