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Ryerson University
Global Management Studies
GMS 401
Kirk Bailey

1S.P.C. Statistical Process Control: GMS-401 F2012 In GMS-401 we study 2 types: 1. Inspection for variables —there is typically one dimension most indicative of QUALITY or lack of Quality of an item being studied for compliance to a Quality Standard. Here it is a dimension such as the contents of a jar of fruit jam, the size of a pair of shoes etc. These are called X-bar and R charts. One calculates X-bar-bar and R-bar averages and these are the centre lines of the SPC run-charts that will be drawn. The charts MUST have these centre lines PLUS upper and lower control AND range limits. The points on these graphs MUST be joined so that a reader can follow the level of quality versus centre lines and control limits over time and look for trends and potential out-of-control conditions. The data will be in a set of readings typically taken at say one-hour intervals. The number of readings taken each hour is the sample size–for example 4 jars of jam in the exercise book. The sample size of 4 is used in calculating the control limits and for determining the value of the statistical constants used in these calculations such as A 2 D 3D 4 The “number of samples” is 10 but the “sample size” is 4. The 10 samples will be plotted on a graph but the number 10 in this case is NOT used in the calculation of control limits when looking up the A ,2 ,3and D v4lues. In this type of SPC BOTH graphs must be drawn and examined. If a SINGLE POINT on either graph exceeds the upper or lower control or range limits, the process is said to be “out of control”. If NO points exceed the range or control limits, the process is said to be “in control. In addition the analyst will examine the graphs for patterns showing either expected random behaviour or the tendency towards an out-of-control condition. The QUALITY GURUS: Deming, Juran, Crosby, Taguchi, Feigenbaum were the originators and proponents of SPC. Their themes were: 1. A statistical sample can be taken and can provide great accuracy as to the quality and acceptability of a batch of production. One need not examine EVERY unit of production–it is too time consuming and too expensive AND would put a firm that used this approach at a competitive disadvantage. 2. Perhaps we cannot have ZERO DEFECTS in production but we can certainly minimize the defects (in many cases a total financial loss for the firm) but taking statistical samples at appropriate intervals (depends on the volatility of the process– how frequently will a factor affect quality–sharpness of tools etc.) and EMPOWERING the production line employees with the ability to shut down the production line if a trend towards OUT-OF-CONTROL is noted. We want employees to stop the process BEFORE it is out-of-control and producing substandard products that will not meet our customer’s needs–and they will probably return them at our expense and stop buying from us. Look at the one-page lime green sheet handed-out in class that discusses how to read and interpret a control chart. There you can see the trends that we would expect production line SPC page 2 of 3 F2012 KLB employees to note and act accordingly—caution if production is heading towards out-of-control and shutting down production before an out-of-control condition occurs and arranging to have the process problems corrected–either by themselves if they work in a TQM (Total Quality Management) factory or calling the Maintenance Department otherwise. 2. Inspection for Attributes: These are p-bar and c-bar charts. p-bar charts are for fraction defective in a batch. (Product is either ok or defective–there is no middle ground). For example 12 coat hangers out of 200 are defective. c-bar charts are defects in a single unit of production–example the rubber sink stoppers in the exercise book. Typically items here are not economically repairable–example standard incandescent light bulb–“a filament enclosed in a glass envelope with a ferrule on the other end–the threaded thing that screws into a light socket”. Firms such as GE and Westinghouse and other firms make these things and test them–they either work or they don’t work–such a lightbulb is a
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