Metrology and Calibration
Metrology underpins every number a quality system reports. Accuracy is closeness to the true value (a systematic property, quantified as bias); precision is closeness of repeated readings (a random property, quantified as repeatability). A gauge can be precise yet inaccurate - a tight cluster far from target. Resolution/discrimination is the smallest readable increment and is not a measure of accuracy.
Calibration compares an instrument's indication to a reference standard and documents the result and its uncertainty; adjustment physically changes the instrument and is a separate act. Expect as-found/as-left data, calibration seals, and status labels. Traceability means an unbroken chain of calibrations to a national or international standard (e.g., NIST) with stated uncertainties, and each link must contribute a small share of total uncertainty - the common 4:1 (or 10:1) test accuracy ratio rule of thumb says the standard should be at least four times more accurate than the unit under test.
Other properties: linearity (error across the range), stability/drift, hysteresis (difference between increasing and decreasing loading), sensitivity, dead band, and environment (20 °C reference temperature, thermal expansion, vibration, humidity). Gauge R&R separates repeatability (equipment) from reproducibility (appraiser).
Out-of-tolerance handling: quarantine the instrument, assess product measured since the last valid calibration, retest or recall, find root cause, and reconsider the calibration interval.
Worked example. A 0–1 in micrometer with 0.001 in resolution requires a standard accurate to at least 0.001/4 = 0.00025 in under a 4:1 TAR; a 0.001 in gage block fails the rule.