XbarR Chart Formulas
QI Macros can calculate them for you!
Create an XbarR Chart using QI Macros
- Select your data.
- Select the chart on QI Macros menu.
- QI Macros will do the math and draw the graph for you.
The XbarR Chart can help you evaluate the stability of processes using variable data--time, cost, length, weight when you have 2 to 10 samples per period.
XbarR Chart formulas used in QI Macros add-in
Average (Xbar) Chart
k = number of subgroups (a group of measurements produced under the same set of conditions)
n = number of samples in a subgroup
A2, D3 and D4 are constants based on n
Conforms with ANSI/ASQC B1, B2, B3 1996
* "Introduction to Statistical Quality Control," Douglas C. Montgomery *
Process: Calculate, plot, and evaluate the Range Chart first. If it is "out of control," so is the process. If the Range Chart looks okay, then calculate, plot, and evaluate the X Chart.
Note: Some people wonder why QI Macros results are a tiny bit different from some versions of other software. The answer is that they use a different estimator. Donald Wheeler in Advanced Topics in Statistical Process Control says: "Even though the Pooled [Standard Deviation] estimator is common in many applications, the robustness of Rbar/d2 and Sbar/c4 makes them the preferred estimators for use with process behavior charts."
Furthermore, Minitab started using Pooled Standard Deviation to calculate Cp/Cpk, and control limits on XbarR and XbarS Charts in versions 15 and 16. Minitab 17 went back to Rbar/d2 and Sbar/c4 for XbarR/S control limits, but retained pooled stdev for Cp/Cpk calculations when using multiple samples.
- Create a XbarR Chart in Excel using QI Macros
- XbarR Chart Video
- XbarR Capability Suite Template in Excel
The X bar R chart is just one of the tools included in QI Macros SPC Software for Excel.
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