Bias is the tendency for a measurement to always be offset on the high or low side of a value. Unfortunately, no standards exist for aligning probe tips on a probe card. The only comparison that takes place is between a supplier's final-inspection measurements of probe tips and the measurements made by a customer at incoming inspection. Differences in measurement results can prove difficult to resolve. Variation is best described as the distribution of results that occurs when you repeatedly measure the same characteristic, on the same item, using the same instrument.
Variation has two components, repeatability and reproducibility.
In simple terms, repeatability indicates an instrument's ability to produce the same results during multiple measurements on the same part. Reproducibility indicates the ability of different operators to produce similar results on the same part. Measurement professionals define these errors in these two measurements in terms of standard deviation of all the measurement values collected for each. And if the ratio exceeds 0.3 then the measurements are unacceptable. These interpretations of the ratio are standards accepted in industry.