Individual X and Moving Range (IX-MR) Chart 

Chart Components

The Inidividual X chart (the upper chart in this figure) illustrates an actual individual reading or measurement taken for quality control sampling purposes. The Moving Range chart (the lower chart in the figure) shows the absolute difference between two consecutive individual values.

IX-MR Charts for a Single Characteristic

Traditionally, an IX-MR chart is used to plot an actual reading and absolute difference between two consecutive readings for a single characteristic. 

For example, this chart (taken from InfinityQS® ProFicient software) represents several batches of resin—a homogeneous mixture. The chart measures the percent solids in each batch. The highlighted plot point shows that for subgroup 16, the moving range plot point exceeds the upper control limit of 0.9.

Individual X and Moving Range Charts for a Single Characteristic

How to Use the IX-MR Chart

Use the Individual X-Moving Range (IX-MR) chart when your sample size is one (n=1). 

By using this chart, you can spot variability that falls outside of what would be considered “normal”—indicating a special cause of variation and a need for investigation and possible process adjustment—for a characteristic, such as percent solids in a homogenous mixture. This is a good chart to use when sampling is expensive, time-consuming, or destructive, or when variation from consecutive samples are likely to indicate a measurement error rather than a variation.

Advantages and Disadvantages of Using the IX-MR Chart

Advantages
  • Easy to understand
  • Need only 15 to 25 individual measurements to estimate control limits
  • Can plot data after obtaining each measurement
  • Requires only minimal calculations
Disadvantages
  • Does not independently separate variation in average from variation in standard deviation
  • Not sensitive enough to quickly identify small changes in process average or standard deviation
  • For some special uses, target origins need to be known for optimal analysis

Decision Tree

Use the following decision tree to determine whether the IX-MR chart is the best choice.
Decision Tree - moving range chart

Special Uses

Today, control charts are a key tool for quality control and figure prominently in Lean manufacturing and Six Sigma efforts.
 

Moving range chart control limits

Target IX-MR Chart

Target charts show multiple characteristics that have different nominal or target values—for example, different specification limits or different tolerances—all on one chart.

In these charts, a zero point represents the target value of each characteristic. Like traditional IX-MR charts, target IX-MR charts help you spot variation in a characteristic. Target charts simply enable you to plot several characteristics in the same chart.
  • Monitor the consistency of a characteristic common to different parts in the same part family.
  • Assess tool wear across multiple runs of different part numbers.
 
See an Example Use Case

Individual moving range chart

Short Run IX-MR Chart

Short run charts accomplish the same goal as target IX-MR charts, but are used for short production runs. These charts combine short run data sets to analyze process capabilities in limited production runs.

  • Detect characteristics that need priority attention.
  • Gain visibility into the difference between variations caused by the process and those that are limited to one product.
 
See an Example Use Case

Moving range chart control limits

Group IX-MR Chart

Group charts display several parameters, characteristics, or process streams on one chart. With a group IX-MR chart, you can assess relative uniformity or consistency across multiple data streams. In the group IX-MR chart, individual measurements and moving ranges from multiple locations are combined into a group. 

  • Clearly and distinctly illustrate the extremes or lack of uniformity in a data set group. 
  • Clearly detect opportunities for improvement.
 
See an Example Use Case

Individual and moving range chart

Group Target IX-MR Chart

As you might expect, the group target IX-MR chart provides the insight of both a group IX-MR chart and a target IX-MR chart. Use this chart to get statistically valid information from multiple part numbers or characteristics that have a common process.

  • Get better visibility into the process instead of into individual parts or test characteristics. 
  • See the difference between variations that are caused by changes in average and those caused by changes in the standard deviation.
 
See an Example Use Case
Moving range chart interpretation

Group Short Run IX-MR Chart 

When you need to evaluate changes in individual measurements across multiple characteristics in a short run environment, use the group short run IX-MR chart.
  • Gain visibility into variations caused by the process as opposed to those that are caused by a specific product, even within short production runs.

  See an Example Use Case

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