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Basics of Process Capability
The document discusses process capability and assessing whether a process is capable of meeting customer requirements. It provides definitions of key terms like capable process, process capability ratios (Cp and Cpk), and discusses the differences between short-term and long-term capability studies. Short-term studies look at random variation over days/weeks using 30-50 data points, while long-term studies examine non-random sources of variation over weeks/months using 100-200 data points. The document warns that capability assessments only indicate potential performance if the process is stable and in control.
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Basics of Process Capability
- 1. 8-1 Is Process Capable ? Is Process Capable ? The Quality Improvement Model Use SPC to Maintain Current Process Collect& Interpret Data Select Measures Define Process Is Process Capable ? Is Process Capable ? Improve Process Capability Is Process Stable ? Investigate & Fix Special Causes No Yes No Yes Is Process Capable? Purpose: Determine the adequacy of the process with respect to customer /management needs.
- 2. 8-2 Is Process Capable ? Is Process Capable ?Capable Process A stableprocess that meets customer requirements. 8 22 242 4 6 10 12 14 16 18 20 26 28 30 32 Run Order UCL CL LCL Control Chart Lower Spec Upper Spec Target Histogram Capability assessments for unstable processes, may not be indicative of how the process is actually performing.
- 3. 8-3 Is Process Capable ? Is Process Capable ? Assessing Process Capability CountingMeasures The average percent defectives. The average number of defects. Instrument Measures Comparing both the center of the process and the process variation
- 4. 8-4 Is Process Capable ? Is Process Capable ? Capability Assessment forCounting Measures “Order Entry Process” Is this process adequate as is? Should it be improved? Week 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Total # Errors 15 22 18 10 13 9 27 12 24 22 8 8 26 16 20 10 16 9 15 20 320 0 5 10 15 20 25 30 35 5 10 15 20 Avg=16.0 LCL=4.0 UCL=28.0 Week Number of Errors Control Chart
- 5. 8-5 Is Process Capable ? Is Process Capable ? Measures of ProcessCapability Cp = Specification Range True Process Range = USL - LSL 6σc Cpk = Distance from process average to closest specification limit 1 2 True Process Range = min (USL - x , x - LSL) 3σc Problem: We are assuming the process has a target that is in the center of the specification range, and that the process is in fact centered on that target. Note: a negative result is possible if the process average is outside specifications Benefits: • Optimal values are attained by running exactly between specs. • Can (must) be used for 1-sided specifications Cp < 1.0 → Process is not capable of meeting specs Cp = 1.0 → Process is marginally capable Cp > 1.0 → Process is capable of meeting specs Cpk < 1.0 → Process is not capable of meeting specs Cpk = 1.0 → Process is marginally capable Cpk > 1.0 → Process is capable of meeting specs Warning: Capability assessments for unstable processes, may not be indicative of how the process is actually performing.
- 6. 8-6 Is Process Capable ? Is Process Capable ? How much materialis out of spec? In the short term? In the long term? USLUSL LSLLSL Process Capability
- 7. 8-7 Is Process Capable ? Is Process Capable ?Process Capability Ratios Voiceof The Customer Voice of The Process USLUSLLSLLSL
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- 9. 8-9 Is Process Capable ? Is Process Capable ? Process Capability -The Strategy Centering –The Process Is On Target Spread – Reduce The Variation LSL USL DefectsDefects
- 10. 8-10 Is Process Capable ? Is Process Capable ? 2 Key Metricsfor Measuring Capability C Min( X-LSL 3 USL-X 3 pk = , ) c LSLUSL Cp σ6 − = cσ cσ Process Capability Ratios
- 11. 8-11 Is Process Capable ? Is Process Capable ?Process Capability Ratios- Concept Total Tolerance C Process Spreadp = c LSLUSL Cp σ6 − =
- 12. 8-12 Is Process Capable ? Is Process Capable ? A Short-term Capabilitystudy covers a relatively short period of time (days, weeks) generally consisting of 30 to 50 data points. The actual number depends on the subject under study. Is The Process In Control ? Is It Producing Defects ? 35302520151050 39 34 29 24 Observation Number IndividualValue I Chart for C1 X=30.60 3.0SL=37.36 -3.0SL=23.84 CP & CPK Measure Short-term Capability
- 13. 8-13 Is Process Capable ? Is Process Capable ? A long-term capabilitystudy covers a relatively long period of time (weeks, months) generally consisting of 100-200 data points. Again, the actual amount depends on the subject under study. Is The Process In Control ? Is It Producing Defects ? 100500 50 40 30 20 Observation Number IndividualValue I Chart for C3 X=33.80 3.0SL=47.12 -3.0SL=20.49 Long Term Performance Short term Capability
- 14. 8-14 Is Process Capable ? Is Process Capable ? A Further Lookat Capability Compare the estimates of the process deviations from the short-term and long-term data What is the difference between the short-term and the long-term data? What implication does this have in doing capability studies? What is the difference between the short-term and the long-term data? What implication does this have in doing capability studies? Descriptive Statistics Variable N Mean StdDev short term 30 30.6 2.23 long term 180 33.8 4.44
- 15. 8-15 Is Process Capable ? Is Process Capable ? Measures of ProcessPerformance Pp = Specification Range True Process Range = USL - LSL 6σs Ppk = Distance from process average to closest specification limit 1 2 True Process Range = min (USL - x , x - LSL) 3σs Problem: We are assuming the process has a target that is in the center of the specification range, and that the process is in fact centered on that target. Note: a negative result is possible if the process average is outside specifications Benefits: • Optimal values are attained by running exactly between specs. • Can (must) be used for 1-sided specifications Pp < 1.0 → Process Performance is not meeting specs Pp = 1.0 → Process Performance is marginally meeting specs Pp > 1.0 → Process Performance is meeting specs Ppk < 1.0 → Process Performance is not meeting specs Ppk = 1.0 → Process Performance is marginally meeting specs Ppk > 1.0 → Process Performance is meeting specs
- 16. 8-16 Is Process Capable ? Is Process Capable ? Performance vs. Capability Thesedata show that the process, if well controlled can perform much better than it currently is 5040302010 14 13 12 11 10 Index CO2-Shrt CO2 Levels for 55 Time PointsDays Sales Outstanding for 55 Days DSO
- 17. 8-17 Is Process Capable ? Is Process Capable ? Capability vs. Performance 5040302010 14 13 12 11 10 Index CO2-Shrt CO2Levels for 55 Time PointsDays Sales Outstanding for 55 Days DSO Process Performance: Total Variation including shifts and drifts (Pp & Ppk) Capability: Only random or short term variability (Cp & Cpk)
- 18. 8-18 Is Process Capable ? Is Process Capable ? Process Performance Ratios TheP-family of indices are computationally the same as the C-family of ‘capability’ indices, but use the observed long-term standard deviation. P Min( X-LSL 3 USL-X 3 pk = , ) s LSLUSL pP σ6 − = sσ sσ
- 19. 8-19 Is Process Capable ? Is Process Capable ? WARNING!!!WARNING!!! Statistical Assumptions MadeIn Capability Studies 1. Data Comes From a Stable ProcessData Comes From a Stable Process If not, work towards getting the process in control Don’t despair, you can still make some assumptions about your process in the mean time