X-Bar R Chart
Classical control chart for continuous measurements in subgroups — X̄ tracks location, R tracks spread.
Also known as: X-Bar R, XBar R chart, mean-range chart
The X̄-R chart combines two charts: the X̄ chart tracks each subgroup's mean; the R chart tracks its range. Location and spread shifts thus appear separately.
- Subgroup size typically n = 4 or 5 (up to 10)
- Factors A_2, D_3, D_4 from tables (depend on n; e.g. A_2 = 0.577 at n = 5)
- Reading order — inspect R first; if spread is out of control the X̄ limits are invalid
- For n > 10 use the X̄-S chart instead (standard deviation rather than range — more efficient for larger subgroups)
See also
Used in
In the Algorithm Lab
Sources
- Shewhart, W. A. — Economic Control of Quality of Manufactured Product, 1931
- Montgomery, D. C. — Introduction to Statistical Quality Control, 8th Edition, Chapter 5
- AIAG — SPC Reference Manual, 2nd Edition