= Systematic error
{tag=Measurement error}
A <systematic error> is a component of <measurement error> that remains constant or changes predictably when measurements are repeated under specified conditions. A ruler with an incorrect scale, a thermometer with a <measurement zero offset>, and an unaccounted <instrumental drift> can all produce such effects. Closely agreeing readings can therefore have poor <measurement accuracy>: repetition tests their scatter, not the correctness of the shared reference.
An estimated <systematic error> should be corrected when possible, with the correction's <measurement uncertainty> included in the <measurement uncertainty budget>. Suppose a balance reads $m+b$ and a reference gives an estimate $\widehat b$ of its offset. The corrected result is $m_{\rm read}-\widehat b$; uncertainty in $\widehat b$ remains even if the same unknown offset affected every weighing. Changing the reference, using a blank, or testing <systematic-error cancellation by reversal> can reveal effects that a long sequence of identical readings misses. The <Type A evaluation of uncertainty> and <Type B evaluation of uncertainty> distinction concerns how uncertainty is evaluated, so neither label is synonymous with systematic error.<Guidelines for Evaluating and Expressing the Uncertainty of NIST Measurement Results by Barry N. Taylor and Chris E. Kuyatt>[Appendix D.1.1–D.1.3: systematic error, correction and uncertainty terminology]{ref}
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