What is an I-MR Chart?

An Individuals and Moving Range (I-MR) chart is a statistical process control (SPC) tool used to monitor continuous data when it is impractical or impossible to collect data in subgroups. It consists of two charts:

• Individuals (I) chart: Displays the individual data points to monitor the process mean over time.
• Moving Range (MR) chart: Displays the absolute difference between consecutive data points to monitor process variation over time.

When to use an I-MR Chart

Use an I-MR chart when:

• Data is continuous (measurements like length, weight, time, temperature).
• Subgrouping is not possible (e.g., destructive testing, batch processing where properties are uniform within a batch, or slow production rates).
• You need to assess both process stability (center and variation) and capability.

How to Interpret I-MR Charts

Always interpret the Moving Range (MR) chart first. If the MR chart is out of control, the control limits on the Individuals (I) chart will be inflated and unreliable, rendering any capability analysis inaccurate.

1. Check Variation (MR Chart): Look for points outside the control limits or non-random patterns. If stable, proceed to the I chart.
2. Check Center (I Chart): Look for points outside the control limits, trends, shifts, or other special cause signals using the Nelson rules (tests).
3. Assess Capability (if stable): Only after establishing that the process is in statistical control should you compare the process output against specification limits (USL/LSL) to determine capability indices (Cp, Cpk, Pp, Ppk).