The I-MR chart is a combined chart consisting of:

·    An Individuals (I) chart, which plots the values of each individual observation, and provides a means to assess process center.

·    A Moving Range (MR) chart, which plots the range calculated from artificial subgroups created from successive observations, and provides a means to assess process variation.

Use an I-MR chart to draw a combined control chart for assessing whether process center and variation are in control when your data are individual observations.

·    An in-control process exhibits only random variation within the control limits.

·    An out-of-control process exhibits unusual variation, which may be due to the presence of special causes.

The MR chart must be in control before you can interpret the I chart because the I chart control limits are calculated considering both process variation and center.

·    When the MR chart is out of control, the control limits on the I chart may be inaccurate and may not correctly signal an out-of-control condition. In this case, the lack of control will be due to unstable variation rather than actual changes in the process center.

·    When the MR chart is in control, you can be sure that an out-of-control I chart is due to changes in the process center.

A liquid detergent company wants to assess whether or not its process is in control. The liquid detergent is made in batches by mixing together a number of ingredients. The quality characteristic of interest is the pH value for each batch. Measurements of the pH value for 25 consecutive batches were taken.

Because the liquid detergent is created in a batch process and only one measurement is taken for each batch, it is not appropriate to place the data in subgroups. Instead, the liquid detergent data should be analyzed using an Individuals (I) control chart, which considers each measurement to be an independent observation.

Data: Detergent.MTW (available in the Sample Data folder).