ASQ - Statistics Division

Using a Graphical Technique for Determining and Displaying Process Capability – Basic Tool Appearing in Summer 1997 Newsletter

Abstract: All statistics vary over time and capability indices such as Cpk are no exception. In order to help establish a more realistic picture of your processes’ capability, one should always use multiple values. By using graphs which show the performance of your process over time (such as an XmR control chart of the Cpk values) one can easily see how the process is performing over time. In addition, knowing the shape of your distribution of individual data by constructing a histogram will help you to better interpret these capability values. We provide an example in which we use the following 3 statistical tools in order to evaluate and display the process capability of a die bond operation: X bar and R control chart, XmR control chart, and histogram.

Keywords: Cpk - I and MR chart - X-bar and R chart

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