Rev. 09/06/01SJSU Bus David Bentley1 Chapter 10 – Quality Control Control process, statistical process control (SPC): X-bar, R, p, c, process capability

Rev. 09/06/01SJSU Bus David Bentley2 SPC – The Control Process Define Measure Compare to a standard Evaluate Take corrective action (if necessary) Evaluate corrective action

Rev. 09/14/01SJSU Bus David Bentley3 Variation Random, chance, or common cause (natural) Assignable, non-random, or special cause (due to some specific change) Sampling used to detect non-random Sampling assumes a normal distribution Sample statistics calculated: Mean Range

Rev. 02/25/04SJSU Bus David Bentley4 Control Charts Control limits When is a process out of control? Charts for variables Mean (X-bar) charts Range (R) charts Charts for attributes Proportion or percentage (p) charts Number [or defects per unit] (c) charts

08/29/02SJSU Bus David Bentley5 Type I and Type II Errors Type I Conclusion: non-randomness is present and the process is out of control Reality: randomness is present and the process is in control Type II Conclusion: randomness is present and the process is in control Reality: non-randomness is present and the process is out of control

Rev. 09/13/01SJSU Bus David Bentley6 Mean & Range Control Charts Take required number of samples Mean (X-bar) charts (see Table 10-2) Calculate mean (X-bar) for each sample Calculate grand mean (X-double-bar) Calculate range (R) for each sample Calculate mean of all sample ranges (R-bar) Calculate UCL and LCL for means Plot grand mean and control limits on X-bar chart

Rev. 09/13/01SJSU Bus David Bentley7 Mean & Range Control Charts Range (R) charts (see Table 10-2) Calculate UCL and LCL for ranges Plot range mean and control limits on R- chart Plot additional samples and determine if within range limits Note: factors based on the size of the sample, not the number of samples!

Rev. 09/06/01SJSU Bus David Bentley8 Run tests and charts Counting above/below the median Counting “ups” and “downs”

01/22/02SJSU Bus David Bentley9 Process Capability Ratios Non-centered process (general case): c pk = the lower of: Upper spec – process mean or 3  Process mean – lower spec 

Rev. 10/01/02SJSU Bus David Bentley10 Process Capability Ratios Centered process (special case): specification width c p = process width Upper spec limit – lower spec limit = 