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X and MR

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Definition

Also called XmR chart or Individual and Moving Range (IMR) chart. A statistical process control chart for use with continuous or count data collected as single observations. It combines two charts; the individual data points are displayed on the top chart and the moving Ranges (xn - Xn-1) displayed on the bottom chart. There are two variations of the XmR chart - one uses the average moving range to calculate the control limits while the other uses the median moving range.

This is the only chart that can be used for discrete as well as continuous data. Transactional processes where there is a single count per period are well suited to this type of control chart - number of orders processed per day, sales amount per day, number of delayed flights per month, etc.

Examples

The store manager at an electronics retail store has charted weekly revenue for the past 4 months. In week 12 the store launched a promotional program offering free DVDs for purchases of above a certain amount. The mR-chart is in control and the X-chart shows a steady increase in sales revenue from week 12 onward, showing that the promotion was successful.

Application

 

External Links

SPC Tutorial - http://www.moresteam.com/toolbox/t402.cfm