About this lessonThe X-bar and Standard Deviation chart is the variable data control chart used when the subgroup is large. This lesson explains how the data is recorded and interpreted on the pair of control charts. The lesson describes how to create this control chart in both Microsoft Excel and using Minitab. The lesson will include practice creating the charts. Show Quick referenceXbarS ChartsThe Xbar-S chart (Subgroup Mean and Standard Deviation) are the variable data control chart to be used when working with a subgroup sample that has ten or more units. It can be easily created in either Microsoft Excel or Minitab. When to useUse the Xbar-S Chart when seeking to control a product or process variable parameter with a high volume process. With high volume and a large number of data points, the standard deviation is a better gauge of the subgroup variation than is the range from max value to min value. The standard deviation uses all the data points and is not as easily impacted by an outlier. InstructionsThe Xbar-S is comprised of a pair of control charts. These are normally aligned so that the Subgroup Mean (Xbar) chart is directly above the Standard Deviation chart. This aids in the recognition of special cause occurrences. The Subgroup Mean chart plots the average value of the data points that were used to sample the subgroup. The Standard Deviation chart plots the value of the standard deviation for all the subgroups. The standard deviation is always a positive number. One other advantage of the Xbar-S chart is that it can work with subgroup sizes that vary slightly. However, they should always be greater than 10 data points. When the subgroup sizes are not equal, the control limit lines will not be straight lines but rather will be stair-stepped. This is because the constants used in the equations change with changing subgroup sample size. Xbar-S Charts can be created in Microsoft Excel or in Minitab. Within Minitab, control charts are created by using the “Stat” pull down menu, then selecting “Control Charts.” Within the Control Charts window, select “Variable Charts for Subgroups” and then finally select “Xbar-S.” In the Minitab Xbar-S Chart panel, you will need to select the data columns with your data. If all the data is in one column, you will need to tell Minitab how many data points are in each subgroup. If all the data is in one column you the data subgroups must have the same number of points. If they don’t you must use the different column approach. If creating the Xbar-S Chart in Excel:
Hints & tips
Login to download
Lesson notes are only available for subscribers. Which type of data is plotted on an X bar and S chart?The Xbar chart plots the mean of the each subgroup. The Xbar charts determine whether the process center is in control. The S chart plots the range of the subgroup standard deviations and is used to determine whether the process variation is in control.
Which type of data is plotted on an XAn X-Bar and R-Chart are control charts utilized with processes that have subgroup sizes of 2 or more. They are a standardized chart for variables data and help determine if a particular process is predictable and stable. These are used to monitor the effects of process improvement theories.
What is XX bar is the average of reading in a sample. S is the standard deviation. S bar is the average of all the standard deviation.
Which type of data is plotted on AC chart attribute data variables data?Attribute control charts are used for attribute data. In other words, the data that counts the number of defective items or the number of defects per unit. For example number of tubes failed on a shop floor. Unlike variable charts, only one chart is plotted for attributes.
|