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Chapter 10

Chapter 10 BU385.docx

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Paul Iyogun

BU385 Chapter 10 – Statistical Quality Control Week 7 Introduction -The best companies emphasize designing quality into the process, thereby greatly reducing the need for inspection/tests -The least progressive companies rely heavily on receiving and shipping inspection/tests -Statistical quality control – use of statistical techniques and sampling in monitoring and testing or quality of goods and services -The part of statistical quality control that relies primarily on inspection/tests of previously produced items is referred to as acceptance sampling -Statistical quality control is important because it provides an economical way to evaluate the quality of products and meet the expectations of customers -Inspection – appraisal of a good or service against a standard Statistical Process Control Planning Process 1. Define the quality characteristics important to customers, and how each is measured 2. For each characteristic a. Determine a quality control point b. Plan how inspection is to be done, how much to inspect, and whether centralized in sit c. Plan the corrective action Define the Quality Characteristics -Define in sufficient detail, what is to be controlled -Different characteristics may require different approached for control purposes -It is important to consider how measurement will be accomplished Determine a Quality Control Point -In manufacturing, some of the typical inspection points are: 1. At the beginning of the process 2. At the end of the process 3. At the operation where a characteristic of interest to customers is first determined How Inspection is to be Done -This is usually technical and needs engineering knowledge -Ex. To test a clarity of beer, a white light is shine through it and its dispersion is measured How Much to Inspect -The amount of inspection can range from no inspection to inspection of each item -Low-cost, high-volume items such as paper clips, nails, and pencils often require little inspection because (1) the cost associated with passing defective items is quite low and (2) the processes that produce these items are usually highly reliable, so that defects are rare -Items that have large costs associated with passing defective products require inspection -The amount of inspection and the expected cost of passing defective items -Operations with a high proportion of human involvement necessitate more inspection than mechanical operations, which tend to be more reliable -A stable process will require only infrequent checks Centralized vs. On-Site Inspection -The central issue in the decision concerning on-site or lab inspections is whether the advantages of specialized lab tests are worth the time and interruption needed to obtain the results -More companies are relying on self-inspections by operators on site BU385 Chapter 10 – Statistical Quality Control Week 7 Plan the Corrective Action -When a process is judged out of control for an important characteristic, the process should be stopped and corrective action must be taken -To ensure that corrective action is effective, the output of a process must be monitored for a sufficient period of time to verify that the problem has been eliminated Statistical Process Control (SPC) -Statistical process control – statistical evaluation of the product in the production process -The operator takes periodic samples from the process and compares them with predetermined limits Types of Variations and Sampling Distributions -Random variation – natural variation in the output of a process, created by countless minor factors -Assignable variation – non-random variability in process output; a variation whose cause can be identified -The main sources of assignable variation can usually be identified and eliminated -The main task of SPC is to distinguish assignable from random variation -Taking a sample of two or more observations and calculating a sample statistic such as ample mean makes the task easier -The sampling distribution of the sample mean can be used to judge whether a process has shifted Control Charts -Control chart- a time-ordered plot of a sample statistic with limits -The basis for a control chart is the sampling distributions -Control limits- the dividing lines for the value of sample statistic between concluding no process shift and a process shift, hence random and assignable variations -The larger value is the upper control limit (UCL), and the smaller value is the lower control limit (LCL) -A sample statistic that falls between these two limits suggests random variation while a value outside or on either limit suggests assignable variation -Control limits are based on the characteristic of the process, whereas the specification limits are based on the desired characteristic of the product -Type I error – concluding that a process has shifted (ex. An assignable variation is present) where it has not (ex. Only random variation is present) -Using wider limits reduces the probability of a Type I error because it decreases the areas in the tails of the distribution -Type II error- concluding that a process has not shifted (ex. Only random variation is present) when it has (Ex. An assignable variation is present) -The costs of making each error should be balanced by their probabilities Designing Control Charts 1. Determine a sample size n 2. Obtain 20 to 25 samples of size n 3. Establish preliminary control limits using appropriate formulas, and graph them 4. Plot the sample statistic values on the control chart, and note whether any points fall outside the control limits 5. If you find no points outside the control limits, assume that there is no assignable cause and therefore the process is stable and in control Sample Mean and Range Control Charts -Sample mean (x) control chart- the control chart for sample mean, used to monitor the process mean BU385 Chapter 10 – Statistical Quality Control Week 7 -The centre line used in the chart represents the process mean -The control limits can be determined in two ways, the choice depends on what information is available -See p. 329 for the formulas if the professor does not give them in slides (UCL and LCL formulas) -The second approach to calculation control limits it to use the sample range (maximum value – minimum value in the sample) -See p. 330 for formulas if the professor does not give them in slides (UCL and LCL formulas) Sample Range Control Chart -Sample range (R) control chart – the control chart for sample range, used to monitor process dispersion or spread -Although the underlying sampling distribution it not Normal, the concept for design and use of sample range control chart is much the same as that f
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