702 CHAPTER 14 Statistical Process Control Run Chart A run chart is one of several different tools commonly used to monitor a process to ensure that desired characteristics don’t change. A run chart is basically the same as a time-series graph, which was introduced in Section 2-3. 14-2 Control Charts for Attributes • Develop the ability to construct a control chart for p, the proportion corresponding to some attribute, such as being a defect. • Identify out-of-control criteria and apply them to determine whether attribute data are within statistical control with only natural variation and no patterns, cycles, or unusual points. Key Concept This section presents run charts, R charts, and x charts as tools that enable us to monitor characteristics of data over time. We can use such charts to determine whether a process is statistically stable (or within statistical control). Process Data The following definition formally describes the type of data that will be considered in this chapter. 14-1 Control Charts for Variation and Mean DEFINITION Process data are data arranged according to some time sequence. They are measurements of a characteristic of goods or services that result from some combination of equipment, people, materials, methods, and conditions. Table 14-1 includes process data consisting of the measured global temperatures 1°C2 of Earth. Because the values in Table 14-1 are arranged according to the time at which they were measured, they are process data. CP EXAMPLE 1 Global Temperatures of Earth as Process Data DEFINITION A run chart is a sequential plot of individual data values over time. One axis (usually the vertical axis) is used for the data values, and the other axis (usually the horizontal axis) is used for the time sequence. Run charts are used to monitor process data for any patterns of changes over time. Continuous process monitoring is critical for identifying problems before they get out of hand. Countless companies have gone bankrupt because they allowed manufacturing processes to deteriorate without constant monitoring. This section introduces three tools commonly used to monitor process data: run charts, R charts, and x charts. We begin with run charts.
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