Instrumental drift
= Instrumental drift
<Instrumental drift> is a change of an instrument's indication or response with <time> while the relevant input is held fixed. A changing <temperature> can alter an electronic offset or the dimensions of a measuring frame; aging can alter its scale. A smooth drift can make repeated readings look precise while shifting the inferred <measurand>.
Interleaving a stable reference with the sample distinguishes a common changing response from a sample change. The <ABBA measurement sequence> cancels a common drift that is an <affine function> of <time> over the sequence; its derivation also shows why curvature in the drift survives. A reference measured only at the start cannot establish that the instrument stayed stable afterward. Residual drift contributes to <measurement uncertainty> rather than disappearing because a correction was attempted.