Statistical Process Control Guardbanding Methods for Inline Optical Screening Systems
Inline optical screening guardbanding absorbs measurement uncertainty into narrowed specification limits to guarantee target consumer risk levels.
Expanded Uncertainty
Meaning
Statistical intervals represent the range of values around a measurement result that contain the true value with a specific level of confidence. Calculation of expanded uncertainty involves multiplying the combined standard uncertainty by a coverage factor, typically denoted as k. This value defines the zone of doubt that remains after all known systematic errors have been corrected.
It is the primary metric for stating the reliability of a calibration or a production test.
Probability Interval
Standard practice in most industrial laboratories uses a coverage factor of two to achieve a confidence level of approximately ninety-five percent. When expanded uncertainty is reported, the factor used must be stated to allow for proper comparison between different test results. A higher factor provides greater confidence but results in a wider interval that may exceed the allowable tolerance window.
Confidence Quantification
Decision makers use this value to assess the likelihood that a part which appears to be within specification is actually outside of it. High expanded uncertainty reduces the effective tolerance available to the manufacturing team. If the interval is too wide, the producer may be forced to reject good parts to ensure that no bad parts are shipped.
This quantification of risk is the difference between a simple measurement and a defensible metrology report.
Tolerance Guardband
The relationship between the tolerance and the measurement range determines the safety margin required for production. Manufacturers often subtract the expanded uncertainty from the upper and lower limits of the specification to create a narrowed acceptance zone. This practice prevents the shipment of borderline items that could fail if remeasured by the customer.
A narrow guardband is only possible when the measurement system is highly stable and well-characterized.