# Statistical Process Control

## Presentation on theme: "Statistical Process Control"— Presentation transcript:

Statistical Process Control
Chapter 3 Statistical Process Control Russell and Taylor Operations and Supply Chain Management, 8th Edition

Lecture Outline Basics of Statistical Process Control – Slide 4
Control Charts – Slide 12 Control Charts for Attributes – Slide 16 Control Charts for Variables – Slide 27 Control Chart Patterns – Slide 45 SPC with Excel and OM Tools – Slide 52 Process Capability – Slide 54 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Learning Objectives Explain when and how to use statistical process control to ensure the quality of products and services Discuss the rationale and procedure for constructing attribute and variable control charts Utilize appropriate control charts to determine if a process is in-control Identify control chart patterns and describe appropriate data collection Assess the process capability of a process © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Statistical Process Control (SPC)
monitoring production process to detect and prevent poor quality Sample subset of items produced to use for inspection Control Charts process is within statistical control limits UCL LCL © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Variability Random Non-Random inherent in a process
depends on equipment and machinery, engineering, operator, and system of measurement natural occurrences Non-Random special causes identifiable and correctable include equipment out of adjustment, defective materials, changes in parts or materials, broken machinery or equipment, operator fatigue or poor work methods, or errors due to lack of training © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

SPC in Quality Management
SPC uses Is the process in control? Identify problems in order to make improvements Contribute to the TQM goal of continuous improvement © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Quality Measures: Attributes and Variables
A characteristic which is evaluated with a discrete response good/bad; yes/no; correct/incorrect Variable measure A characteristic that is continuous and can be measured Weight, length, voltage, volume © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

SPC Applied to Services
Nature of defects is different in services Service defect is a failure to meet customer requirements Monitor time and customer satisfaction © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

SPC Applied to Services
Hospitals timeliness & quickness of care, staff responses to requests, accuracy of lab tests, cleanliness, courtesy, accuracy of paperwork, speed of admittance & checkouts Grocery stores waiting time to check out, frequency of out-of-stock items, quality of food items, cleanliness, customer complaints, checkout register errors Airlines flight delays, lost luggage & luggage handling, waiting time at ticket counters & check-in, agent & flight attendant courtesy, accurate flight information, cabin cleanliness & maintenance © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

SPC Applied to Services
Fast-food restaurants waiting time for service, customer complaints, cleanliness, food quality, order accuracy, employee courtesy Catalogue-order companies order accuracy, operator knowledge & courtesy, packaging, delivery time, phone order waiting time Insurance companies billing accuracy, timeliness of claims processing, agent availability & response time © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Where to Use Control Charts
Process Has a tendency to go out of control Is particularly harmful and costly if it goes out of control Examples At beginning of process because of waste to begin production process with bad supplies Before a costly or irreversible point, after which product is difficult to rework or correct Before and after assembly or painting operations that might cover defects Before the outgoing final product or service is delivered © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Charts A graph that monitors process quality Control limits
upper and lower bands of a control chart Attributes chart p-chart c-chart Variables chart mean (x bar – chart) range (R-chart) © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Control Chart Out of control Upper control limit Process
average Lower control limit 1 2 3 4 5 6 7 8 9 10 Sample number © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Normal Distribution Probabilities for Z= 2.00 and Z = 3.00 =0 1 2
3 -1 -2 -3 95% 99.74% © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

A Process Is in Control If …
… no sample points outside limits … most points near process average … about equal number of points above and below centerline … points appear randomly distributed © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Charts for Attributes
p-chart uses portion defective in a sample c-chart uses number of defects (non-conformities) in a sample © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

p-Chart UCL = p + zp LCL = p - zp
z = number of standard deviations from process average p = sample proportion defective; estimates process mean p = standard deviation of sample proportion p = p(1 - p) n © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Construction of p-Chart
20 samples of 100 pairs of jeans NUMBER OF PROPORTION SAMPLE # DEFECTIVES DEFECTIVE : : : 200 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Construction of p-Chart
total defectives total sample observations p = = p(1 - p) n UCL = p + z = UCL = p(1 - p) n LCL = p - z = LCL = © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Construction of p-Chart
total defectives total sample observations p = = 200 / 20(100) = 0.10 p(1 - p) n 0.10( ) 100 UCL = p + z = UCL = 0.190 p(1 - p) n 0.10( ) 100 LCL = p - z = LCL = 0.010 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Construction of p-Chart
© 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

p-Chart in Excel Click on “Insert” then “Charts” to construct control chart I4 + 3*SQRT(I4*(1-I4)/100) I4 - 3*SQRT(I4*(1-I4)/100) Column values copied from I5 and I6 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

c-Chart UCL = c + zc c = c LCL = c - zc where
c = number of defects per sample c = c © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

c-Chart UCL = c + zc LCL = c - zc
Number of defects in 15 sample rooms NUMBER OF DEFECTS SAMPLE c = : : 190 UCL = c + zc LCL = c - zc © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

c-Chart Number of defects in 15 sample rooms 15 c = = 12.67 1 12 2 8
: : 190 SAMPLE c = = 12.67 15 UCL = c + zc = = 23.35 LCL = c - zc = = 1.99 NUMBER OF DEFECTS © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

c-Chart © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Charts for Variables
Range chart ( R-Chart ) Plot sample range (variability) Mean chart ( x -Chart ) Plot sample averages © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart:  Known = UCL = x + z x LCL = x - z x = Where - - - =
x1 + x xk k X = = process standard deviation x = standard deviation of sample means =/ k = number of samples (subgroups) n = sample size (number of observations) n © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Known
Observations(Slip-Ring Diameter, cm) n - Sample k x We know σ = .08 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Known
x1 + x xk k X = = - UCL = x + z x = - LCL = x - z x = - © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Known
= (.08 / ) 10 = 4.93 _____ 50.09 = 5.01 X = = LCL = x - z x - UCL = x + z x = (.08 / ) = 5.09 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Unknown
_ UCL = x + A2R LCL = x - A2R = where x = average of the sample means R = average range value © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Chart Factors n A2 D3 D4 2 1.880 0.000 3.267
Factors for R-chart Sample Size Factor for X-chart © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Unknown
OBSERVATIONS (SLIP- RING DIAMETER, CM) SAMPLE k x R Totals © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Unknown
_ R = ____ ∑ R k UCL = x + A2R = x = x ___ LCL = x - A2R © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x-bar Chart Example:  Unknown
1.15 10 R = = = 0.115 _ ____ _ _ UCL = x + A2R = 50.09 10 _____ x = = = 5.01 cm x k ___ = (0.58)(0.115) = 5.08 LCL = x - A2R = (0.58)(0.115) = 4.94 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

x- bar Chart Example © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

R- Chart UCL = D4R LCL = D3R R k R = Where R = range of each sample
k = number of samples (sub groups) © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

R-Chart Example Totals OBSERVATIONS (SLIP- RING DIAMETER, CM)
SAMPLE k x R Totals © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Retrieve chart factors D3 and D4
R-Chart Example Retrieve chart factors D3 and D4 UCL = D4R = LCL = D3R = _ © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Retrieve chart factors D3 and D4
R-Chart Example Retrieve chart factors D3 and D4 UCL = D4R = 2.11(0.115) = 0.243 LCL = D3R = 0(0.115) = 0 _ © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

R-Chart Example © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

X-bar and R charts – Excel & OM Tools
© 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Using x- bar and R-Charts Together
Process average and process variability must be in control Samples can have very narrow ranges, but sample averages might be beyond control limits Or, sample averages may be in control, but ranges might be out of control An R-chart might show a distinct downward trend, suggesting some nonrandom cause is reducing variation © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Chart Patterns
Run sequence of sample values that display same characteristic Pattern test determines if observations within limits of a control chart display a nonrandom pattern © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Chart Patterns
To identify a pattern look for: 8 consecutive points on one side of the center line 8 consecutive points up or down 14 points alternating up or down 2 out of 3 consecutive points in zone A (on one side of center line) 4 out of 5 consecutive points in zone A or B (on one side of center line) © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Chart Patterns
UCL UCL LCL LCL Sample observations consistently below the center line Sample observations consistently above the center line © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Control Chart Patterns
LCL UCL Sample observations consistently increasing consistently decreasing © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Zones for Pattern Tests
UCL LCL Zone A Zone B Zone C Process average 3 sigma = x + A2R = 3 sigma = x - A2R 2 sigma = x (A2R) 2 3 2 sigma = x (A2R) 1 sigma = x (A2R) 1 1 sigma = x (A2R) x Sample number | 4 5 6 7 8 9 10 11 12 13 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Performing a Pattern Test
B — B B U C B D A B D A B U C — U C A U C A U B A U A A D B SAMPLE x ABOVE/BELOW UP/DOWN ZONE © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Sample Size Determination
Attribute charts require larger sample sizes 50 to 100 parts in a sample Variable charts require smaller samples 2 to 10 parts in a sample © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

SPC with Excel © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

SPC with OM Tools © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Capability Compare natural variability to design variability
What we measure with control charts Process mean = 8.80 oz, Std dev. = 0.12 oz Tolerances Design specifications reflecting product requirements Net weight = 9.0 oz  0.5 oz Tolerances are  0.5 oz © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Design Specifications
Process Capability (b) Design specifications and natural variation the same; process is capable of meeting specifications most of the time. Design Specifications Process (a) Natural variation exceeds design specifications; process is not capable of meeting specifications all the time. © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Design Specifications
Process Capability (c) Design specifications greater than natural variation; process is capable of always conforming to specifications. Design Specifications Process (d) Specifications greater than natural variation, but process off center; capable but some output will not meet upper specification. © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Capability Ratio
Cp = tolerance range process range upper spec limit - lower spec limit 6 = © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Computing Cp Net weight specification = 9.0 oz  0.5 oz
Process mean = 8.80 oz Process standard deviation = 0.12 oz Cp = upper specification limit - lower specification limit 6 Computing Cp © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Computing Cp Net weight specification = 9.0 oz  0.5 oz
Process mean = 8.80 oz Process standard deviation = 0.12 oz Cp = = = 1.39 upper specification limit - lower specification limit 6 6(0.12) © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Capability Index
Cpk = minimum x - lower specification limit 3 = upper specification limit - x , © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Computing Cpk Net weight specification = 9.0 oz  0.5 oz
Process mean = 8.80 oz Process standard deviation = 0.12 oz = x - lower specification limit 3 , Cpk = minimum = upper specification limit - x 3 © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Computing Cpk Net weight specification = 9.0 oz  0.5 oz
Process mean = 8.80 oz Process standard deviation = 0.12 oz Cpk = minimum = minimum , = 0.83 x - lower specification limit 3 = upper specification limit - x , 3(0.12) © 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Capability With Excel
© 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e

Process Capability With OM Tools
© 2014 John Wiley & Sons, Inc. - Russell and Taylor 8e