For example, let’s say you are taking a measurement with a specification limit of 5 volts and the measurement system being used to measure a DUT has a measurement accuracy of 1 volt. To make sure that a DUT with a true value greater than 5 volts does not pass, the limit should be guard-banded by the 1 volt measurement accuracy and set at 4 volts.
Here is how this works. You have a DUT with a true value of 5 volts that could be measured anywhere from 4 to 6 volts due to measurement accuracy of 1 volt. See Figure 1.
Now let’s say that there was a DUT with a true value of 5.5 volts. We would never want this DUT to pass but it would pass 25% of the time. See Figure 2.