Chart Control Limits

Control limits are represented by a line above and below the center line, and labelled as UCL (upper control limit) and LCL (lower control limit). The values for these limits are based on certain calculations and determines the color of the line to display.

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.