I-MR Control Chart (Individual-Moving Range)
For processes with one measurement per time point (batch processes, destructive tests, low-volume). I chart monitors individual values; MR chart monitors variation between consecutive points.
When only one sample is taken per measurement (no rational subgroups), the I-MR individuals & moving range chart is the right choice. It suits non-destructive single-piece measurements (resistance, hardness, thickness), directly monitoring whether each unit stays within control limits. Process engineers use I-MR charts to track slow long-term drift, catching tool wear or material lot differences early before mass defects occur.
Measurement series (in time order): 10.2, 9.8, 10.3, 9.7, 10.1, 10.5, 10.8... Moving range MR: |10.2-9.8|=0.4, |9.8-10.3|=0.5, |10.3-9.7|=0.6... MR-bar = mean moving range = 0.45 I-chart control limits: UCL = X̄ + 3×(MR̄/1.128) = 10.2 + 3×0.399 = 11.40 LCL = X̄ - 3×(MR̄/1.128) = 10.2 - 1.197 = 9.00 → Point 10.8 is near the UCL — watch the trend