Shewhart-CUSUM Control Chart (Unified Guidance)

See Also:

Shewhart-CUSUM Control Chart (1989 Guidance)

Shewhart-CUSUM Control Chart Options (Unified Guidance)

Description:

The Unified Guidance provides a variation on the 1989 guidance for Shewhart-CUSUM control charts. The principles are the same, with two main differences:

1. The Unified Guidance plots non-standardized concentrations as opposed to the standardized concentrations of the 1989 guidance.

2. The Unified Guidance compares plotted values only to the non-standardized h line, rather than the standardized h and SCL lines.

Additionally, as implemented in ChemStat, the Unified Guidance method does not support Fast Initial Response (FIR).

Although described in the Unified Guidance as a method for inter-well comparisons, as implemented in ChemStat, it can be used for intra-well and inter-well comparisons.

Use:

As a graphical method for intra-well or inter-well comparisons.

Options:

Select Control Chart Options Unified Guidance from the Options menu or from the right-mouse-button context menu to enter estimates of h and k. Also entered from this option is the number of baseline measurements. If a positive number of baseline measurements is entered, the baseline measurements are counted from the first sampling date. If a negative number is entered, the number of baseline measurements is equal to the total number of measurements for the well minus the absolute value of the number entered. Baseline measurements are not graphed, but are used only to establish baseline mean and standard deviation.

Total Indicated Baseline Measurements

Measurements Baseline Measurements Measurements Graphed

15 4 4 11

15 -4 11 4

The line style connecting points for the Z line and the S line can also be specified from Control Chart Options Unified Guidance.

Remarks:

Shewhart-CUSUM control charts may be very susceptible to false positives and are not robust with respect to outlier values. However, the graph is useful in providing a visual indication of changing trends in parameter concentrations.

Note that in specifying the number of baseline measurements, it is the actual number of measurements that is counted and not the number of sampling dates. However the graph plots a single point for each sampling date, regardless of the number of measurements collected on that date.