Statistical process control

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Statistical process control

  1. 1. S TATISTICALPROCESS CONTROL CUSTOMER & COMPETITIVE INTELLIGENCE FORPRODUCT, PROCESS, SYSTEMS & ENTERPRISE EXCELLENCE DEPARTMENT OF STATISTICS REDGEMAN@UIDAHO.EDU OFFICE: +1-208-885-4410 DR. RICK EDGEMAN, PROFESSOR & CHAIR – SIX SIGMA BLACK BELT
  2. 2. Quality Management:Statistical Process Control
  3. 3. Statistical Process Control Statistical Process Control (SPC) can be thought of as the application of statistical methods for the purposes of quality control and improvement. Quality Improvement is perhaps foremost among all areas in business for application of statistical methods.
  4. 4. Data Driven Decision Making © “In God we trust. ... all others must bring data.” --- The Statistician’s Creed © SPC is one method that assists in enabling “data-driven decision making”. © SPC is a key quantitative aid to quality improvement efforts.
  5. 5. Control Charts: Recognizing Sources of Variation• Why Use a Control Chart? – To monitor, control, and improve process performance over time by studying variation and its source.• What Does a Control Chart Do? – Focuses attention on detecting and monitoring process variation over time; – Distinguishes special from common causes of variation, as a guide to local or management action; – Serves as a tool for ongoing control of a process; – Helps improve a process to perform consistently and predictably for higher quality, lower cost, and higher effective capacity; – Provides a common language for discussing process performance.
  6. 6. Control Charts:Recognizing Sources of Variation • How Do I Use Control Charts? – There are many types of control charts. The control charts that you or your team decides to use should be determined by the type of data that you have. – Use the following tree diagram to determine which chart will best fit your situation. Only the most common types of charts are addressed.
  7. 7. Control Chart Selection: Variable DataMeasured & Plotted on a Continuous Scale such as Time, Temperature, Cost, Figures. n=1 2<n<9 n is ‘small’ n is ‘large’ median 3<n<5 n > 10 X & Rm X&R X&R X&S
  8. 8. Control Chart Selection: Attribute Data Counted or Plotted as Discrete Events Such as Shipping Errors, Waste or Absenteeism. Defect or Defective Data Nonconformity Data Constant Variable Constant Variable sample size sample size n > 50 n > 50 c chart u chart p or np chart p chart
  9. 9. Control Chart Construction• Select the process to be charted;• Determine sampling method and plan; – How large a sample needs to be selected? Balance the time and cost to collect a sample with the amount of information you will gather. – As much as possible, obtain the samples under the same technical conditions: the same machine, operator, lot, and so on. – Frequency of sampling will depend on whether you are able to discern patterns in the data. Consider hourly, daily, shifts, monthly, annually, lots, and so on. Once the process is “in control”, you might consider reducing the frequency with which you sample. – Generally, collect 20-25 groups of samples before calculating the statistics and control limits. – Consider using historical data to establish a performance baseline.
  10. 10. Control Chart Construction• Initiate data collection: – Run the process untouched, and gather sampled data. – Record data on an appropriate Control Chart sheet or other graph paper. Include any unusual events that occur.• Calculate the appropriate statistics and control limits: – Use the appropriate formulas.• Construct the control chart(s) and plot the data.
  11. 11. Control Chart Interpretation:Time, Production & Spatial Analysis: Still-Life Photography © An event taken in isolation or a group of items each selected from a process during the same (brief) time span can generally provide information about process performance ONLY during that brief span. © Unless process performance is static through time this will be true. © Dynamic processes vary through time.
  12. 12. Control Chart Interpretation:Time, Production & Spatial Analysis: The Video Generation © If a process varies through time, it is often useful to know how the process varies so that it can be “controlled” or “guided” in its behavior. © This requires monitoring through time, similar to videotaping the process - in some sense, the process has a “life of its own” and we want to nurture that life.
  13. 13. Control Chart Interpretation: Persistence Through Time• A process can be characterized by: – Examining its behavior during a sufficiently brief interlude of time – Examining its behavior across a greater expanse of time.• Stable process: one which performs with a high degree of consistency at an essentially constant level for an extended period of time – “In-control”• A process that is not stable is referred to as being in an out- of-control state
  14. 14. Data Plot with PAT Zones 36.10 (A)36 34.36 (B)34 32.62 (C)32 30.8830 29.14 (C)28 27.40 (B)26 25.66 (A) 0 5 10 15 20 25 30 Item
  15. 15. Control Chart Interpretation: Pattern Analysis Tests (PATs)• PAT 1: One point plots beyond zone A on either side of the mean• PAT 2: Nine points in a row plot on the same side of the mean• PAT 3: Six consecutive points are strictly increasing or strictly decreasing• PAT 4: Fourteen consecutive points which alternate up and down
  16. 16. Control Chart Interpretation: Pattern Analysis Tests• PAT 5: Two out of three consecutive points plot in zone A or beyond, and all three points plot on the same side of the mean• PAT 6: Four out of five consecutive points plot in zone B or beyond, and all five points plot on the same side of the mean
  17. 17. Control Chart Interpretation: Pattern Analysis Tests• PAT 7: Fifteen consecutive points plot in zones C, spanning both sides of the mean• PAT 8: Eight consecutive points plot at more than one standard deviation away from the mean with some smaller than the mean and some larger than the mean
  18. 18. Control Chart Interpretation: Monitoring & Improving Processes• The performance of every process will be composed of two primary components: – Controlled or guided performance which is predictable in both an instantaneous and long-term sense – Uncontrolled variation • Special or assignable causes • Common causes
  19. 19. Control Chart Interpretation: Monitoring & Improving Processes• True process improvement is typically a result of either: – Breakthrough thinking – Efforts to identify and reduce or eliminate common causes of variation; methodical quantitatively oriented tools which monitor a process over time --- the approach taken generally by “control charts”.
  20. 20. Control Chart Interpretation• The vertical axis coordinate of a point plotted on the chart corresponding to the value of an appropriate PPM and the horizontal axis coordinate of a point plotted on the chart corresponding to the time in sequence at which the observation was made with the time between observations divided into equal increments.
  21. 21. Control Charts: Colors Used UCL A * U2SWL B * * U1SL ** C * * * * CL * * C * * L1SL* B * L2SWL A LCL
  22. 22. P Charts for the Process Proportion Based on m preliminary samples from the process. While the number of items, n, may vary from sample to sample, it is customary for each of the samples in a given application to include the same number of items, n. For the ith of these m samples, let Yi = number of defective units in the sample Then the proportion defective for the ith sample is: pi = Yi / ni
  23. 23. Control Chart Interpretation• Center line (CL) positioned at the estimated mean• Upper and lower one standard deviation lines (U1SL and L1SL) positioned one standard deviation above and below the mean.• Upper and lower two standard deviation warning lines (U2SWL and L2SWL) positioned at two standard deviations above and below the mean.• Upper and lower control lines (UCL and LCL) positioned at three standard deviations above and below the mean.
  24. 24. P Charts for the ProportionAn estimate of the overall process proportion defective is p = (Y1+Y2+...+ Ym) / (n1+n2+...+ nm) = (total defectives) / (total items)When all samples have n items each then p = (p1 + p2 + ... + pm)/mThe estimated standard deviation of the process proportiondefective is Sp = √ p (1-p)/ ni
  25. 25. P Chart Control Lines & LimitsThe coordinates for the seven lines on the Pchart are positioned at:CL = pU1SL = p + Sp L1SL = p - SpU2SWL = p + 2Sp L2SWL = p - 2SpUCL = p + 3Sp LCL = p - 3Sp
  26. 26. South of the Borders, Inc. Custom Wallpapers & Borders Free Estimates (013) 555-9944
  27. 27. South of the Borders, Inc.South of the Borders, Inc. is a custom wallpapers and bordersmanufacturer. While their products vary in visual design, themanufacturing process for each of the products is similar.Each day a sample of 100 rolls of wallpaper border issampled and the number of defective rolls in the sample isnoted.The number of defective rolls in samples from 25 consecutiveproduction days follows.Determine all coordinates; construct & interpret the p chart. PATs 1, 2, 3 and 4 apply to p charts.
  28. 28. South of the Borders, Inc.Day Defective Rolls Day Defective Rolls 1 13 14 8 2 4 15 9 3 7 16 3 4 11 17 5 5 8 18 14 6 10 19 10 7 2 20 11 8 9 21 6 9 12 22 6 10 6 23 9 11 4 24 3 12 7 25 10 13 9
  29. 29. South of the Borders, Inc. Total # of items sampled = 2500 Total # of defective items = 196 p = 196/2500 = .0784 Sp = √ .0784(.9216)/100 = .02688
  30. 30. South of the Borders, Inc.CL = .0784UCL = .0784 + 3(.0269) = .1590LCL = .0784 - .0806 = -.0022 (na)U2SWL = .0784 + 2(.0269) = .1322L2SWL = .0784 - .0538 = .0246U1SL = .0784 + .0269 = .1053L1SL = .0784 - .0269 = .0515
  31. 31. P Chart for Defective Wallpaper Rolls 3.0SL=0.1590 0.15 2.0SL=0.1322Proportion 1.0SL=0.1053 0.10 P=0.07840 0.05 -1.0SL=0.05152 -2.0SL=0.02464 0.00 -3.0SL=0.000 Subgroup 0 5 10 15 20 25 Rolls 8 6 9 11 10 Proportion of Defective Rolls Received
  32. 32. South of the Borders, Inc. P Chart Interpretation• No violations of PATs one through four are apparent. This implies that the process is “in a state of statistical control”.• It does not indicate that we are satisfied with the performance of the process.• It does, however, indicate that the process is stable enough in its performance that we may seriously engage in PDCA for the purpose of long-term process improvement.
  33. 33. C and U Charts for Nonconformities• When data originates from a Poisson process, it is customary to monitor output from the process with a defects or C chart• Recall the Poisson Distribution with mean = c and standard deviation = √c• P(y) = cye-c/y!
  34. 34. C & U Charts for Nonconformities• C represents the average number of defects (nonconformities) per measured unit with all units assumed to be of the same “size” and all samples are assumed to have the same number of units• m = 20 to 40 initial samples• C = (number of defects in the m samples) / m• Estimated standard deviation= √C
  35. 35. C Control Chart Coordinates• CL = C• UCL = C+3 C and LCL = C-3 C• U2SWL= C+2 C and L2SWL = C- 2 C• U1SL = C+ C and L1SL = C- C
  36. 36. Scientific & Technical Materials, Inc.
  37. 37. Scientific & Technical Materials, Inc. • Scientific & Technical Materials, Inc. produces material for use as gaskets in scientific, medical, and engineering equipment. Scarred material can adversely affect the ability of the material to fulfill its intended use. • A sample of 40 pieces of material, taken at a rate of 1 per each 25 pieces of material produced gave the results on the following slide. Use this information to construct and interpret a C chart.
  38. 38. Scientific & Technical Materials, Inc. Piece 1 2 3 4 5 6 7 8 9 10 Scars 4 4 2 3 1 2 0 2 3 1 Piece 11 12 13 14 15 16 17 18 19 20 Scars 1 1 2 3 0 4 3 2 2 1 Piece 21 22 23 24 25 26 27 28 29 30 Scars 2 1 0 3 5 4 2 1 4 2 Piece 31 32 33 34 35 36 37 38 39 40 Scars 2 1 1 3 2 0 1 5 9 1
  39. 39. Scientific & Technical Materials, Inc. • C= 90/40= 2.25= CL, Sc= 2.25 = 1.5 • UCL= 2.25+ 3(1.5) = 6.75 • LCL= 2.25- 4.5 = -2.25 (NA) • U2SWL= 2.25+ 2(1.5)= 5.25 • L2SWL= 2.25- 3 = -0.75 (NA) • U1SL= 2.25+ 1.5 = 3.75 • L1SL= 2.25- 1.5 = 0.75
  40. 40. Scientific & Technical Materials, Inc. C Chart for Gasket Material Data10987 UCL6 U2SWL54 U1SL3 CL21 L1SL01 3 5 7 9 25 11 13 15 17 19 21 23 27 29 31 33 35 37 39
  41. 41. Scientific & Technical Materials, Inc. C Chart Interpretation• Application of PATs one through four indicates a violation of PAT 1 at sample number 39 where 9 scars appear on the surface of the sampled material.• Corrective measures would be identified and implemented.• After process stability was (re) assured, we would move into PDCA mode.
  42. 42. Variation of the C chart where Sample size may varyU = (u1+u2+...+um) / (n1+n2+...+nm) = (total # of defects) / (total # of units in the m samples)• CL = U• UCL = U+ 3 U/ni, LCL= U-3 U/ni• U2SWL= U+ 2 U/ni, L2SWL= U- 2 U/ni• U1Sl= U+ U/ni, L1SL= U- U/ni U Chart
  43. 43. Control Charts for the Process Mean and Dispersion‘X bar’ ChartTypically used to monitor process centrality (or location)Limits depend on the measure is used to monitor process dispersion(R or S may be used).‘S’ or ‘Standard Deviation’ Chart:Used to monitor process dispersion‘R’ or ‘Range’ Chart:Also used to monitor process dispersion
  44. 44. Sample Summary Information• m = 20 to 40 initial samples of n observations each.• Xi = mean of ith sample• Si = standard deviation of ith sample• Ri = range of ith sample X = (X1 + X2 +... + Xm) / m R = (R1 + R2 + ... +Rm)/m S = (S1 + S2 + ... + Sm)/m σ = R/d2 where d2 depends only on n
  45. 45. Coordinates for the X-bar Control Chart: “R” • CL= X, • UCL= X+ A2R, • UCL= X- A2R • U2SWL= X+ 2A2R/3 • L2SWL= X- 2A2R/3 • U1SL= X+ A2R/3 • L1SL= X- A2R/3 A2 is a constant that depends only on n.
  46. 46. Coordinates for an R Control Chart• CL= R• UCL= D4R• LCL= D3R• U2SWL= R+ 2(D4-1)R/3• L2SWL= R- 2(D4-1)R/3• U1SL= R+ (D4-1)R/3• L1SL= R- (D4-1)R/3• where D3 and D4 depend only on n
  47. 47. Championship Card Company Championship
  48. 48. Championship Card Company Championship Card Company (CCC) produces collectible sports cards of college and professional athletes. CCCs card-front design uses a picture of the athlete, bordered all-the-way-around with one-eighth inch gold foil. However, the process used to center an athlete’s picture does not function perfectly. Five cards are randomly selected from each 1000 cards produced and measured to determine the degree of off-centeredness of each card’s picture. The measurement taken represents percentage of total margin (.25”) that is on the left edge of a card. Data from 30 consecutive samples is included with your materials, and summarized on the following slides.
  49. 49. Championship Card CompanySample X-bar R Sample X-bar R Sample X-bar R 1 55.6 22 11 51.2 15 21 50.0 11 2 61.0 23 12 49.4 14 22 47.0 14 3 45.2 20 13 44.0 32 23 50.6 15 4 46.2 11 14 51.6 14 24 48.8 16 5 46.8 18 15 53.2 12 25 44.6 22 6 49.8 23 16 52.4 23 26 46.8 16 7 46.8 18 17 50.6 8 27 49.2 8 8 44.2 20 18 56.0 18 28 45.6 19 9 50.8 32 19 50.2 19 29 57.6 40 10 48.4 16 20 44.0 23 30 51.4 17
  50. 50. Championship Card Company Summary Information n=5 A3 = 1.427 X = 49.63 B3 = NA S = 7.42 B4 = 2.089 R = 18.63 D3 = NA d2 = 2.326 D4 = 2.115 A2 = 0.577 σ = R/d2 = 8.01
  51. 51. Championship Card CompanyX-bar and R Control Chart Limits X based on R R UCL 60.38 39.40 U2SWL 56.80 32.48 U1SL 53.22 25.55 CL 49.63 18.63 L1SL 46.05 11.71 L2SWL 42.47 4.79 LCL 38.89 ------
  52. 52. Championship Card Company X Bar Chart for Sports Cards Centering Values Limits Based on R 1 60 3.0SL=60.38 2.0SL=56.80 S m Me n a ple a 1.0SL=53.22 50 X=49.63 - 1.0SL=46.05 - 2.0SL=42.47 40 - 3.0SL=38.89 0 10 20 30 Sample Number Samples of 5 from each 1000 Cards Printed
  53. 53. Championship Card Company R Chart for Sports Card Centering 40 3.0SL=39.40 2.0SL=32.48 30 a ple a geS m Rn 1.0SL=25.55 20 R=18.63 - 1.0SL=11.71 10 - 2.0SL=4.791 0 - 3.0SL=0.000 0 10 20 30 Sample Number Samples of 5 Cards from each 1000 Produced
  54. 54. Championship Card Company X-bar & R Chart Interpretation• Application of all eight PATs to the X-bar chart indicated a violation of PAT 1 (one point plotting above the UCL) at sample 2. Apparently, a successful process adjustment was made, as suggested by examination of the remainder of the chart.• Application of PATs one through four to the R chart indicated a violation of PAT 1 at sample 29. Measures would be investigated to reduce process variation at that point. The violation was a “close call” and was out of character with the remainder of the data.• We are close to being able to apply PDCA to the process for the purpose of achieving lasting process improvements.
  55. 55. Coordinates for the X bar Control Chart: “S” • CL= X • UCL= X= A3S • LCL= X- A3S • U2SWL= X+ 2A3S/3 • L2SWL= X- 2A3S/3 • U1SL= X+ A3S/3 • L1SL= X- A3S/3 • where A3 depends only on n
  56. 56. Coordinates on an S Control Chart • CL= S • UCL= B4S • LCL= B3S • U2SWL= S+ 2(B4-1)S/3 • L2SWL= S- 2(B4-1)S/3 • U1SL= S+ (B4-1)S/3 • L1SL= S- (B4-1)S/3 • where B3 and B4 depend only on n
  57. 57. Championship Card CompanySample X-bar S Sample X-bar S Sample X-bar S 1 55.6 9.63 11 51.2 6.83 21 50.0 5.15 2 61.0 8.63 12 49.4 5.46 22 47.0 5.15 3 45.2 7.40 13 44.0 14.35 23 50.6 5.55 4 46.2 4.09 14 51.6 5.18 24 48.8 6.50 5 46.8 7.22 15 53.2 5.36 25 44.6 8.96 6 49.8 8.76 16 52.4 9.48 26 46.8 6.50 7 46.8 6.72 17 50.6 3.44 27 49.2 3.19 8 44.2 8.53 18 56.0 7.00 28 45.6 7.96 9 50.8 11.95 19 50.2 7.60 29 57.6 14.38 10 48.4 6.19 20 44.0 8.46 30 51.4 6.80
  58. 58. Championship Card Company X-bar and S Chart Limits X based on S SUCL 60.22 15.49U2SWL 56.69 12.80U1SL 53.16 10.11CL 49.63 7.42L1SL 46.11 4.72L2SWL 42.58 2.03LCL 39.05 ------
  59. 59. Championship Card Company X Bar Chart for Sports Cards Centering Values 1 Limits Based on S 60 3.0SL=60.22 2.0SL=56.69S m Me n a ple a 1.0SL=53.16 50 X=49.63 - 1.0SL=46.11 - 2.0SL=42.58 40 - 3.0SL=39.05 0 10 20 30 Sample Number Samples of 5 from each 1000 Cards Printed
  60. 60. Championship Card Company S Chart for Sports Card Centering Values 15 3.0SL=15.49 2.0SL=12.80Sm S v a ple tde 10 1.0SL=10.11 S=7.416 5 - 1.0SL=4.724 - 2.0SL=2.032 0 - 3.0SL=0.000 0 10 20 30 Sample Number 5 Cards Sampled from each 1000 Cards Produced
  61. 61. Championship Card Company X-bar & S Chart Interpretation• Application of all eight PATs to the X-bar chart indicates a violation of PAT 1 (one pt. above the UCL) at sample 2. Judging from the remainder of the chart, the process was successfully adjusted.• Application of the first four PATs to the S chart indicates no violations.• In summary, the process appears to have been temporarily “out-of-control” w.r.t. its mean at sample 2. The process was successfully adjusted and may now be subjected to PDCA for permanent improvement purposes.
  62. 62. Common Questions for Investigating an Out-of-Control Process• Are there differences in the measurement accuracy of instruments / methods used?• Are there differences in the methods used by different personnel?• Is the process affected by the environment, e.g. temperature/humidity?• Has there been a significant change in the environment?• Is the process affected by predictable conditons such as tool wear?• Were any untrained personnel involved in the process at the time?• Has there been a change in the source for input to the process such as a new supplier or information?• Is the process affected by employee fatigue?
  63. 63. Common Questions for Investigating an Out-of-Control Process• Has there been a change in policies or procedures such as maintenance procedures?• Is the process frequently adjusted?• Did the samples come from different parts of the process? Shifts? Individuals?• Are employees afraid to report “bad news”?
  64. 64. Process Capability:The Control Chart Method for Variables Data 1. Construct the control chart and remove all special causes. NOTE: special causes are “special” only in that they come and go, not because their impact is either “good” or “bad”. 3. Estimate the standard deviation. The approach used depends on whether a R or S chart is used to monitor process variability. ^ _ ^ _ σ = R / d2 σ = S / c4 Several capability indices are provided on the following slide.
  65. 65. Process Capability Indices: Variables Data ^ ^ CP = (engineering tolerance)/6σ = (USL – LSL) / 6σ This index is generally used to evaluate machine capability. tolerance to the engineering requirements. Assuming that the process is (approximately) normally distributed and that the process average is centered between the specifications, an index value of “1” is considered to represent a “minimally capable” process. HOWEVER … allowing for a drift, a minimum value of 1.33 is ordinarily sought … bigger is better. A true “Six Sigma” process that allows for a 1.5 σ shift will have Cp = 2.
  66. 66. Process Capability Indices: Variables Data ^ ^CR = 100*6σ / (Engineering Tolerance) = 100* 6 σ /(USL –LSL) This is called the “capability ration”. Effectively this is the reciprocal of Cp so that a value of less than 75% is generally needed and a Six Sigma process (with a 1.5σ shift) will lead to a CR of 50%.
  67. 67. Process Capability Indices: Variables Data ^ ^ CM = (engineering tolerance)/8σ = (USL – LSL) / 8σ This index is generally used to evaluate machine capability. Note … this is only MACHINE capability and NOT the capability of the full process. Given that there will be additional sources of variation (tooling, fixtures, materials, etc.) CM uses an 8σ spread, rather than 6σ. For a machine to be used on a Six Sigma process, a 10σ spread would be used.
  68. 68. Process Capability Indices: Variables Data = ^ = ^ZU = (USL – X) / σ ZL = (X – LSL) / σZmin = Minimum (ZL , ZU)Cpk = Zmin / 3 This index DOES take into account how well or how poorlycentered a process is. A value of at least +1 is required with avalue of at least +1.33 being preferred.Cp and Cpk are closely related. In some sense Cpk represents thecurrent capability of the process whereas Cp represents thepotential gain to be had from perfectly centering the process
  69. 69. Process Capability: ExampleAssume that we have conducted a capability analysis using X-bar and Rcharts with subgroups of size n = 5. Also assume the process is instatistical control with an average of 0.99832 and an average range of0.02205. A table of d2 values gives d2 = 2.326 (for n = 5). Suppose LSL =0.9800 and USL = 1.0200^ _σ = R / d2 = 0.02205/2.326 = 0.00948Cp = (1.0200 – 0.9800) / 6(.00948) = 0.703CR = 100*(6*0.00948) / (1.0200 – 0.9800) = 142.2%CM = (1.0200 – 0.9800) / (8*(0.00948)) = 0.527ZL = (.99832 - .98000)/(.00948) = 1.9ZU = (1.02000 – .99832)/(.00948) = 2.3 so that Zmin = 1.9
  70. 70. Process Capability: InterpretationCp = 0.703 … since this is less than 1, the process is not regarded as being capable.CR = 142.2% implies that the “natural tolerance” consumes 142% of the specifications (not a good situation at all).CM = 0.527 = Being less than 1.33, this implies that – if we were dealing with amachine, that it would be incapable of meeting requirements.ZL = 1.9 … This should be at least +3 and this value indicates that approximately 2.9% of product will be undersized.ZU = 2.3 should be at least +3 and this value indicates that approximately 1.1% of product will be oversized.Cpk = 0.63 … since this is only slightly less that the value of Cp the indication is that there is little to be gained by centering and that the need is to reduce
  71. 71. S TATISTICALPROCESS CONTROL End of Session DEPARTMENT OF STATISTICS REDGEMAN@UIDAHO.EDU OFFICE: +1-208-885-4410 DR. RICK EDGEMAN, PROFESSOR & CHAIR – SIX SIGMA BLACK BELT

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