cl05_Quality_Management_II

assignable cause variation systematic errors

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Unformatted text preview: nated only by ________________ Assignable cause variation (systematic errors, abnormal variability) variability) – Special or assignable cause – A “discoverable” cause discoverable cause – Can be eliminated by ______________________ 10 Stephen Shum Spring 2009 In Control VS Out of Control In Control VS Out of Control A process is in control if process is in control if it it is operating without __________ cause __________ cause variation. Oth Otherwise, the process is th out out of control, and and immediate corrective immediate corrective action action can be taken and will be fruitful. 11 Stephen Shum Spring 2009 Statistical Statistical Process Control: Control Charts Ch Process Parameter Upper Control Limit (UCL) Center Line = Process Average Track process parameter over time Distinguish between – Common cause variation Common cause variation (within (within the control limits) – Assignable cause variation (outside the control limits) Lower Control Limit (LCL) Time 12 Stephen Shum Spring 2009 Statistical Statistical Process Control: Control Charts Ch Process Parameter Upper Control Limit (UCL) When observation falls When observation falls outside outside control limit… – You must take action – You must take action determine determine cause and correct if necessary Center Line = Process Average Lower Control Limit (LCL) Is ti hter control always tight better? – Chance of false alarm of false alarm Out of control Time 13 Stephen Shum Spring 2009 Control Control Charts: Determining Control Limits Li Strike a balance: mean +/- 3σ +/– – – – – 3σ is taken to be “natural variation” 3 in 1,000 are out of control Out of control observations do occur, but not that all that often A compromise between sensitivity and errors Probability of false alarm < 0.0027 14 Mean 95.5% 99.73% Stephen Shum Spring 2009 Control Control Charts: Variable vs. Attribute Att Variable control chart Variable control chart (x-chart) – For product characteristics that can be measured – Example: length, size, we...
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