Chart types

The SPC Chart control supports displaying characteristics of all types. The following is the list of chart types supported for specific characteristic type:

• A Binary Attribute characteristic can be displayed as a PercentDefective (p) chart or NumberDefective (np) chart.

• A Counted Attribute characteristic can be displayed as a NumberOfDefects (c) chart, DefectsPerUnit (u) chart, or DefectsPerMillionOpportunities (DPMO) chart.

• A Variable characteristic can be displayed as a XBarRange (XBarR) chart, XBarSigma (XBarS) chart, MovingAverageMovingRange (MARange) chart, MovingAverageSigma (MASigma) chart, or XIndividualRange (IxIr) chart.

Depending on the chart type and configuration of the variable characteristic, the following are the additional limitations:

PercentDefective (p) and DefectsPerUnit (u)

The p and u charts are ratio charts that can have a variable sample size in the denominator. As such, it is recommended to set control limits with either standard values or auto-calculated.

If preset limits are used, the chart requires the "single point outside control limits" rule to be enabled so that the fixed control limits are applied to the chart. If the QM specification does not have this rule enabled, the SPC Chart control will enable the rule on the chart only. The rule checking within the MES database will not use the single point outside control limits rule, unless the QM Specification has this rule enabled. However, the chart will, which can lead to inconsistencies in what is recorded in the MES database and what is shown in the SPC Chart.

X Bar

This chart plots samples that have multiple results per sample. If all the samples for the characteristic have only one reading per sample, a blank chart is displayed.

X Bar Range

This chart is designed for fixed sample sizes. However, the chart does support plotting samples where the number of results in the sample is less than or greater than the standard sample size. Because the sample size is artificially the same, all samples will have the same control limits applied to them. When using fixed control limits, the chart supports asymmetric upper and lower limits. When using standard values or auto-calculated control limits, the chart will have symmetric upper and lower limits.

X Bar Sigma

This chart does support variable sample sizes so the control limits can change from point to point. To accomplish this, the chart sets limits based on a center line and standard mean (Use standard values option). If the characteristic being plotted has fixed limits, these limits are transferred into a center line and a standard mean value for the upper control limit and for the lower control limit. The chart supports asymmetric upper and lower control limits.

Moving Average

This chart starts plotting with the first result received and adjusts the control limits accordingly until the number of points plotted is equal to the number of results used in the moving average calculation. If two or more samples to be plotted have the same sample request time, then one second is added to the times for each plotted point after the first.

Determining and Displaying Control Limits

• For all variable characteristic chart types, the control limits are calculated by the chart based on the center line and standard deviation.

• An upper and lower sigma are supported, so fixed limits can be unbalanced. The upper sigma is calculated by subtracting the mean from the upper control limit and dividing by 3. The lower sigma is calculated by subtracting the lower control limit from the mean and dividing by 3. The normal sample size does not factor into this equation. If either the upper control limit, lower control limit, or mean are missing, the control limits are automatically calculated from the data.

• When the characteristic definition has single-side control limits, the control rule violations displayed in the chart might not reflect the control rules calculated by the MES system [recorded in the MES database and reported to the Sample Recording Object (SRO) if one exists].

• When calculating the lower control limit for an attribute or range chart, a negative calculated value is not displayed and does not factor into calculated control rule violations.