Slides_Ch 16

Slides_Ch 16 - Introduction to Statistical Quality Control...

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Chapter 16 1 Introduction to Statistical Quality Control (SQC) Synonym : Statistical Process Control ( SPC ) It is based on the premise that some degree of variability in manufacturing processes is inevitable. SQC is widely used for discrete parts manufacturing (e.g., automobiles, microelectronics) and product quality control. SQC References: 1. Text, 4/e; Chapter 16. pp. 645-62. 2. Seborg, Edgar and Mellichamp, Process Dynamics and Control, 2 nd ed ., Chapter 21, Wiley, NY (2004) Note: Equation, figure, and table numbers that begin with “21” are from Ref. 2.
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Chapter 16 2 Attempts to distinguish between two types of situations: 1. Normal operation and “chance causes”, that is, random variations. 2. Abnormal operation due to a “special cause” (often unknown). SQC uses “Quality Control Charts” (also called, “Control Charts”) to distinguish between normal and abnormal situations. Basic Model for SQC monitoring activity: x = μ + ε where x is the measured value, μ is its (constant) mean, and ε is a random error. Introduction to SQC (continued)
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Chapter 16 3 It is important to distinguish between the theoretical mean, , and the sample mean, . If measurements of a variable are normally distributed, , the sample mean is also normally distributed. μ x () 2 μ , σ N The Normal Distribution (review)
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Chapter 16 4 The Control Chart x The most widely used control chart is the chart. This type of control chart is often referred to as a Shewhart Chart, in honor of the pioneering statistician, Walter Shewhart, who first developed it in the 1920s. x In SQC, control charts are used to determine whether the process operation is normal or abnormal. Two general types of control charts: 1. Charts for measures of central tendency (e.g., sample mean, individual measurements) 2. Charts for measures of variability (e.g., standard deviation, range) Control Charts
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Chapter 16 5 Example 21.1 A manufacturing plant produces 10,000 plastic bottles per day. Because the product is inexpensive and the plant operation is normally satisfactory, it is not economically feasible to inspect every bottle. Instead, a sample of n bottles is randomly selected and inspected each day. These n items are called a subgroup , and n is referred to as the subgroup size . The inspection includes measuring the toughness of x of each bottle in the subgroup and calculating the sample mean . x Example of an Control Chart x x
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Chapter 16 6 Figure 21.4 The control chart for Example 21.1. x
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Chapter 16 7 The control chart in Fig. 21.4 displays data for a 30-day period. The control chart has a target ( T ), an upper control limit ( UCL ), and a lower control limit ( LCL ). The target (or centerline ) is the desired (or expected ) value for while the region between UCL and LCL defines the range of normal variability, as discussed below.
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Slides_Ch 16 - Introduction to Statistical Quality Control...

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