Median-Range Control Chart: A Robust Variables Control Chart
What is the Median-Range Control Chart?
The median-range chart plots the subgroup median instead of the mean for location and the subgroup range for dispersion. Because the median is unaffected by extreme values, it is more robust than Xbar-R when data contain outliers, mild skew, or are recorded by hand on the shop floor. The arithmetic is simple enough for operators to calculate and interpret directly.
When to Use It
Use it for variables data with subgroup sizes of 2–10, occasional outliers or non-ideal distributions you prefer not to transform, and manual measurement and recording where frontline staff must read the chart. For subgroups larger than 10 the median loses efficiency — use Xbar-s instead; for individual observations use I-MR.
How to Use It (Step by Step)
Paste observations in subgroups (one row per subgroup) and the tool looks up the A4, D3 and D4 constants, draws the median and range charts, and marks any rule violations. Read the range chart first and then the median chart, as with Xbar-R, and investigate special causes with a 4M1E approach.
Key Formulas / Example
Median chart limits: X̃ ± A4·R̄ where A4 depends on n (n = 5 → 0.691); range chart: UCL = D4·R̄ and LCL = D3·R̄ (D3 = 0 for n ≤ 6). Example: n = 5 and R̄ = 2.1 gives UCL_R = 2.114 × 2.1 ≈ 4.44.