I-MR Control Chart: SPC for Single-Value Data
What is an I-MR Chart?
The I-MR chart combines an individuals chart (I) and a moving range chart (MR): the I chart plots each measured value directly and the MR chart plots the absolute difference between consecutive values. When each subgroup has only one observation, the moving range substitutes for the within-subgroup range to estimate process variation.
When to Use It
Use it for variable data where only one value can be taken at a time: destructive testing (one piece tested), automated gauges outputting single readings, very low-volume processes (a few parts per day), or single-point sampling in continuous production. It is the standard SPC tool for individual-value data.
How to Read It
The MR chart reflects short-term variation between consecutive samples; an out-of-control MR means excessive run-to-run variation. The I chart reflects the overall process level; an out-of-control I means the mean has shifted or a special cause exists. Read the MR chart first, then the I chart - the tool marks signals automatically.
Control Limits
I chart: Xbar +/- E2*MRbar, where E2 is tied to the moving-range span (usually taken as 2.66 for a range of 2). MR chart: UCL = D4*MRbar and LCL is typically 0. The tool performs all calculations and draws both charts.