Control Charts for Variables

Introduction

Many quality characteristics can be expressed in terms of a numerical measurement. A single measurable quality characteristic, such as a dimension, weight, or volume, is called a Variable. Control charts for variables are used extensively. They usually lead to more efficient control procedures and provide more information about process performance than attributes control charts.

When dealing with a quality characteristic that is a variable, it is a standard practice to control both the mean value of the quality characteristic and its variability. Control of the process average or mean quality level is usually with the control chart for means, or the x bar chart. The control of the process range is done by using the control chart for range, or the R chart.

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X bar and R charts

The X bar chart is developed from the average of each subgroup data. The R chart is developed from the ranges of each subgroup data, which is calculated by subtracting the maximum and the minimum value in each subgroup. An example showing the development of the charts is provided.


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