Inside GLP 9: rounding is a metrological act

Rounding feels like arithmetic hygiene, but done carelessly it quietly moves results and pass/fail decisions. NIST's GLP 9 — two dense pages in the library — treats rounding as part of the measurement, with rules that keep it from becoming a bias.

The core rules are familiar (round half to even, round once at the end, never round intermediate values), but the reasons are metrological: repeated half-up rounding biases averages upward; rounding before combination discards information the uncertainty analysis needs; and rounding a value more finely than its uncertainty supports manufactures false precision.

The GUM's reporting convention closes the loop: state the expanded uncertainty to two significant figures and round the result to match — which is exactly what the significant-figures calculator in our Tools section implements. GLP 9 is the one-page reading to hand a new technician before their first certificate.