The analytical tools of Six Sigma have been used for many years in traditional q
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The analytical tools of Six Sigma have been used for many years in traditional quality improvement programs. What makes their application to Six Sigma unique is the integration of these tools in a corporatewide management system. The tools common to all quality efforts are flowcharts, run charts, Pareto charts, histograms, checksheets, cause-and-effect diagrams, and control charts. Examples of these, along with an opportunity flow diagram, are shown in Exhibit 12.5, arranged according to DMAIC categories where they commonly appear. Flowcharts. There are many types of flowcharts. The one shown in Exhibit 12.5 depicts the process steps as part of a SIPOC (supplier, input, process, output, customer) analysis. SIPOC in essence is a formalized input-output model, used in the define stage of a project. Run charts. They depict trends in data overtime, and thereby help in understanding the magnitude of a problem at the define stage. Typically, they plot the median of a process. Pareto charts. These charts help to break down a problem into the relative contributions of its components. They are based on the common empirical finding that a large percentage of problems are due to a small percentage of causes. In the example, 80 percent of customer complaints are due to late deliveries, which are 20 percent of the causes listed. Checksheets. These are basic forms that help standardize data collection. They are used to create histograms such as shown on the Pareto chart. Cause-and-effect diagrams. Also called fishbone diagrams, they show hypothesized relationships between potential causes and the problem under study. Once the C&E; diagram is constructed, the analysis would proceed to find out which of the potential causes were in fact contributing to the problem. Opportunity flow diagram. This is used to separate value-added from non-value-added steps in a process. Process control charts. These are time-sequenced charts showing plotted values of a statistic including a centerline average and one or more control limits. It is used to assure that processes are in statistical control. Why is the R chart used in conjunction with Xbar chart? Describe the purpose of each, and why not just use a P chart?Explanation / Answer
An Xbar chart is used to plot the process mean and R chart is used to plot the process range for the subgroups. This combination of Xbar and R charts is used for examining the stability of processes.
The Xbar chart and R chart both should are interpreted together to determine the stability of process. R chart tells whether the process variation is in control, then only we can correctly interpret the Xbar chart. The control limits of Xbar chart are calculated based on both the process spread and the center. If R chart is out of control, then control limits of Xbar will be inaccurate and may falsely indicate or may even fail to detect an out-of-control situation.
We can use a P chart only where countable defects are involved. A P-chart cannot be used to examine a process having measurable and not countable variables.
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