XbarS Chart Formulas
The XbarS Chart can help you evaluate the stability of processes using variable data--time, cost, length, weight when you have 6 or more samples per period.
Here are the XbarS Chart formulas used in QI Macros for both the Standard Deviation and Average (Xbar) Charts:
XbarS Standard Deviation Chart Formula
Average (Xbar) Chart Formula
k = number of subgroups (a group of measurements produced under the same set of conditions)
n = number of samples in a subgroup
A3, B3 and B4 are constants based on n
Conforms with ANSI/ASQC B1, B2, B3 1996
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 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 XbarS Chart in Excel using QI Macros
- XbarS Chart Video
- XbarS Capability Suite Template in Excel
The XbarS Chart is just one of the tools included in the QI Macros for Excel SPC Software for Excel.
Why Choose QI Macros Over Other Control Chart Software?
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