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A measurement method describes the general logical arrangement of operations used to make a measurement. A measurement procedure supplies the detailed instructions for implementing that arrangement with a particular measuring system. The VIM lists substitution measurement methods, null measurement methods, differential methods, and direct or indirect methods among the possible descriptions.^[ref]

For an original substitution measurement method, drive an unknown resistor with a fixed electric current $I$ and record a voltmeter indication. Replace it by an adjustable reference resistor without changing $I$ or the other relevant conditions, and adjust the reference until the indication agrees. In an ideal measurement model,so matching $v_x=v_s$ implies $R_x=R_s$. The common additive readout offset $v_0$ cancels. This cancellation depends on the current, connections and offset remaining common; the name of the method alone does not guarantee those conditions.

A null measurement method instead adjusts a reference to make a difference signal vanish, as in a balanced Wheatstone bridge. A resistance inferred from separately measured electric potential difference and electric current uses the relation $R=V/I$. These are descriptions of how the information is obtained; the operational record must still specify apparatus, settings, timing, reference values and the measurement uncertainty. The measurement principle concerns the physical phenomenon used, while the measurement method concerns the organization of the measurement. Different methods can exploit the same physical principle.

A measurement principle is the phenomenon on which a measurement is based. The VIM gives the thermoelectric effect used to measure temperature as a physical example.^[ref] Naming that phenomenon identifies the physical connection that makes an observable signal informative about the measurand.

A thermocouple uses the Seebeck effect to produce an electric potential difference associated with temperatures along a circuit of dissimilar conductors. In a chosen approximation with constant coefficient $S$, the measurement model can be writtenwhere $T_r$ is the reference-junction temperature. For an original ideal example, choose $S=40\mu \mathrm{V}{\mathrm{K}}^{-1}$, $T_r=20{}^{\circ }\mathrm{C}$ and $V=2.00\mathrm{mV}$. The inferred temperature difference is $50\mathrm{K}$, so $T_h=70{}^{\circ }\mathrm{C}$. These stipulated values illustrate the model, not the calibration of a particular thermocouple type.

The reference condition matters physically: the same signal at $T_r=25{}^{\circ }\mathrm{C}$ would imply $T_h=75{}^{\circ }\mathrm{C}$ in this model. A readout that neglects the reference temperature can give a precise-looking indication without measuring the intended temperature correctly. The measurement principle explains why the signal arises; it does not by itself supply all corrections or the measurement uncertainty.

The measurement method describes how operations are organized around the principle, for example comparing an output with a reference. The measurement procedure specifies the actual conductors, junction arrangement, temperature references, readout settings, settling interval and calculations. Different measuring systems can share a measurement principle while implementing different methods and procedures.

Shared bias in an interlaboratory consensus occurs when a common systematic error shifts several laboratories' results together. Their interlaboratory consensus value can then be highly reproducible while displaced from an independent reference. Shared calibration standards, a common correction or the same inadequately modeled measurement step can create such dependence. The GUM explicitly warns that using the same instrument, standard or reference datum can correlate input quantities.^[ref]

A simple original model iswhere $\theta$ is the target quantity value, $b$ a common fixed offset and ${\epsilon }_i$ a laboratory-specific deviation. The arithmetic mean is $\bar{x}=\theta +b+\bar{\epsilon }$. Increasing the number of independent laboratories reduces the fluctuations in $\bar{\epsilon }$ but leaves $b$ in place.

The obstacle is also one of identification. For any constant $c$, replacing $(\theta ,b)$ with $(\theta +c,b-c)$ leaves all predicted results unchanged. Pairwise differences $x_i-x_j$ cancel both terms. Thus the comparison's internal agreement cannot determine the absolute common offset without additional information.

To represent residual common-reference uncertainty, use an uncertain zero-centered quantity $B$ with standard uncertainty $u_B$, independent of mutually independent laboratory errors of variance ${\sigma }^2$. ThenThis is a stated uncertainty model, distinct from asserting that an unknown fixed bias has been observed to vary randomly. The common contribution is counted once in the mean's uncertainty; it is not divided by the number of laboratories.

For stipulated $u_B=0.50$, $\sigma =0.20$ and $n=25$, treating everything as independent would retain only $0.20/5=0.04$. The stated correlated model instead gives $u(\bar{x})=\sqrt{0.25+0.0016}\approx 0.502$, in the same units as the measurement. The appropriate covariance terms prevent the implausible conclusion that enough copies of a shared reference can remove its uncertainty.^[ref]

An independently characterized certified reference material, a different measurement principle or an independently calibrated reference can help test the suspected common shift. Independence must itself be checked: a nominally separate reference derived from the same calibration chain may retain the same dependency. Recognized biases should be corrected where appropriate, with the correction's uncertainty retained.

WIKA is a manufacturer of measuring instruments whose technical publications describe pressure measurements using elastic sensing elements. Its technical information distinguishes pressure-driven deformation from the linkage and pointer mechanism that turns it into a measurement indication.^[ref] Its documentation is useful for connecting a physical measurement principle to the specifications of a complete instrument.

A concrete specification also illustrates the difference between an error limit and measurement uncertainty. WIKA's June2025 US sheet for specified four- and six-inch gauges gives a base limit of $\pm 1.0\%$ of span and an additional temperature error of $\pm 0.4\%$ of span per $10\mathrm{K}$ away from the $20{}^{\circ }\mathrm{C}$ reference.^[ref]

For an original instrument example with span $1\mathrm{MPa}$, the base bound is $10\mathrm{kPa}$. A $20\mathrm{K}$ temperature departure contributes a further bound $8\mathrm{kPa}$. Adding those magnitudes gives a conservative $18\mathrm{kPa}$ combined bound under this simplified interpretation; it is not a statistical root-sum-square uncertainty calculation. At a reading of $100\mathrm{kPa}$, even the base bound is $10\%$ of the reading, despite being $1\%$ of span.

The example concerns the stated version and conditions, not a brand-wide accuracy claim. Use the actual model, range and pressure gauge temperature error specification together with instrument calibration evidence when interpreting a particular Bourdon tube pressure gauge.