Designing the overlap between instruments

Replacing an instrument can improve measurement while disrupting a long record. This methods note synthesises GCOS change-management principles, NIST traceability policy, the JCGM uncertainty guide, and Arctic observing-network context to examine what an overlap period must establish. A fictional temperature comparison demonstrates how a small mean offset can conceal a scale-dependent difference. The analysis treats condition coverage, measurand equivalence, time alignment, correlated pairs, uncertainty in both instruments, and reversible corrections as distinct design questions. It separates calibration, numerical continuity, and validation, and explains the additional limits when simultaneous comparison was impossible. No universal overlap duration, sample count, or calibration procedure is proposed. The aim is a bounded statement of comparability supported by preserved evidence before retirement makes that evidence unobtainable.