Measurement Uncertainty & Error Analysis Flashcards

What is an uncertainty budget in calibration?

Answer: A tabulated list of all uncertainty sources with their magnitudes and combined contribution

An uncertainty budget systematically lists every source of uncertainty, its magnitude, distribution type, sensitivity coefficient, and contribution to the combined standard uncertainty.

Repeatability in measurement is defined as:

Answer: Closeness of agreement between repeated measurements under the same conditions in a short time period

Repeatability describes the closeness of agreement among successive measurements of the same measurand carried out under identical conditions (same operator, instrument, location, and time).

Reproducibility in measurement differs from repeatability in that it:

Answer: Assesses agreement under varied conditions such as different operators, locations, or instruments

Reproducibility evaluates measurement agreement when one or more conditions are changed (different operator, lab, or equipment), making it a broader measure of consistency than repeatability.

The Welch-Satterthwaite equation in uncertainty analysis is used to:

Answer: Calculate the effective degrees of freedom for the combined uncertainty

The Welch-Satterthwaite formula calculates the effective degrees of freedom of the combined uncertainty, which is needed to determine the appropriate coverage factor for a given confidence level.

Guard bands in calibration decision-making are used to:

Answer: Reduce the acceptance zone to account for measurement uncertainty and control false acceptance risk

Guard bands narrow the acceptance zone by the measurement uncertainty to reduce the probability of accepting out-of-tolerance instruments (false acceptance), as described in ILAC-G8 and ASME B89.7.3.1.

Measurement traceability in calibration means that a result can be:

Answer: Related to national or international measurement standards through an unbroken chain of comparisons with stated uncertainties

Traceability requires an unbroken chain of calibrations linking a measurement result back to an international or national standard (e.g., NIST), with each link having a documented uncertainty.

When calibration uncertainty is evaluated using a rectangular (uniform) probability distribution, the standard uncertainty is calculated as:

Answer: Half-width of the interval divided by √3

For a rectangular distribution with half-width a, the standard deviation (standard uncertainty) is a/√3, because the variance of a uniform distribution over [-a, +a] is a²/3.