Robust Xbar-S chart

This macro creates an Xbar-S chart that is robust to the presence of diffuse and localized disturbances; either a shift affecting specific subgroups or extreme individual observations within a subgroup.

Download the Macro

Be sure that Minitab knows where to find your downloaded macro. Choose File > Options > General. Under Macro location browse to the location where you save macro files.

Important

If you use an older web browser, when you click the Download button, the file may open in Quicktime, which shares the .mac file extension with Minitab macros. To save the macro, right-click the Download button and choose Save target as.

Download RobustChart.mac

Required Inputs

A column of numeric observations
A corresponding column of subgroup indicators

Note

The use of this version of the macro is limited to subgroups of size 5, 7, 9, 11, 13, or 15. As to the number of subgroups, the macro assumes the user will have either 20, 30, or 40. If one of these conditions is not met, the macro will produce an error message and close. This is because the unbiasing constants needed to calculate a robust estimate of sigma are currently only available for these values.

Optional Inputs

EXCLUDE K: Use to specify a different percentage for the trimmed means (default is 20%).
TUNING K: Use to include other tuning parameters. Only the unbiasing constants 7 and 10 have been included as part of this macro.

Running the Macro

Suppose your observations are in C1 and the subgroup IDs are in C2. To run the macro, choose View > Command Line/History and type:

%ROBUSTCHART C1 C2

Click Run.

More Information

Because Phase I estimates are assumed to be calculated from a process that is stable and under statistical control, robust parameter estimates need to be computed in the presence of special-cause variation. For more technical details about the algorithm, refer to the references below:

Tatum, L.G. (1997). "Robust Estimation of the Process Standard Deviation for Control Charts", Technometrics, Vol. 39(2): 127-141.

Schoonhoven, M., Nazir, H.Z., Riaz, M., and Does, R.J.M.M. (2011). "Robust Location Estimators for the Xbar Control Chart", JQT. Vol. 43(4): 363-379.