Chap010 - Chapter 10 Quality Control CHAPTER 10 QUALITY...

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Chapter 10 - Quality Control CHAPTER 10: QUALITY CONTROL Teaching Notes As a result of increased global competition, a rapidly growing number of companies of all sizes are paying much more attention to issues involving quality and productivity. Many statistical techniques are available to assist organizations in improving the quality of their products and services. It is important for companies to use these techniques in the context of an overall quality system (Total Quality Management) which requires quality awareness, careful planning and commitment to quality at all levels of the organization. Many companies are not only utilizing these statistical techniques themselves, but are also requiring their suppliers to meet certain standards of quality based on various statistical measures. This chapter covers the statistical applications of quality control. Control charts are given the primary emphasis, but other quality control topics such as process capability and inspection are also discussed. When covering the material in this chapter, we need to stress that through the use of control charts, the nonrandom (special) causes of variation must be controlled before random (common) causes of variation and process capability can be analyzed. Answers to Discussion and Review Questions 1. The elements in the control process are: a. Define b. Measure c. Compare to standard d. Evaluate e. Take corrective action if needed f. Evaluate corrective action to insure it is working 2. Control charts are based on the premise that a process which is stable will reflect randomness: statistics of samples taken from the process (means, number of defects, etc.) will conform to a sampling distribution with known characteristics, so that statistical significance tests can be performed on sample statistics; and successive samples will not reveal any patterns which will enable prediction of future values other than specification of range of variability. 3. Control charts are used to judge whether the sample data reflects a change in the parameters (e.g., mean) of the process. This involves a yes/no decision and not an estimation of process parameters. 4. Order of observation of process output is necessary if patterns (e.g., trends, cycles) in the output are to be detected. 5. a. x chart—A control chart used to monitor process variables by focusing on the central tendency of a process. b. Range control charts are used to monitor process variables, focusing on the dispersion of a process. 10-1
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Chapter 10 - Quality Control c. p-chart—is a control chart for attributes, used to monitor the proportion of defectives in a process. d. c-chart—is a control chart for attributes, used to monitor the number of defects per unit. 6. A run is a sequence of observations with a given characteristic. Run tests are helpful in detecting patterns in time series (e.g., control chart) data.
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