I‑MR Chart

The I‑MR chart (Individuals and Moving Range) is one of the most flexible and widely used control charts in the Control phase. It is ideal for processes where subgrouping is not practical—such as low‑volume production, long cycle times, or transactional processes where data arrives one point at a time. The I‑MR chart provides a clear, intuitive view of both the process level and its short‑term variation. 

The Individuals (I) chart plots each measurement over time. It shows the central tendency of the process and highlights shifts, trends, and out‑of‑control points. Because each point represents a single observation, the chart is sensitive to special causes and provides immediate visibility into process behavior. 

The Moving Range (MR) chart plots the absolute difference between consecutive observations. It reflects short‑term variation—the natural “bounce” between one measurement and the next. The MR chart helps determine whether the process variation is stable and whether the control limits on the I chart are reliable. 

One of the strengths of the I‑MR chart is its simplicity. It requires no subgrouping and minimal assumptions. It works well with skewed data, irregular sampling intervals, and processes where data is collected manually. This makes it ideal for service environments, healthcare, administrative processes, and custom manufacturing. 

However, the I‑MR chart requires careful interpretation. Because each point is an individual measurement, the chart can appear noisy. Patterns such as trends, cycles, or clustering must be evaluated thoughtfully. The MR chart is especially important for validating stability; if the MR chart is out of control, the I chart’s limits may be unreliable. 

In the Control phase, the I‑MR chart provides a practical, accessible way to monitor stability in processes where subgrouping is not feasible. It helps teams detect special causes early, maintain control, and sustain improvements over time.