CCT Measurement Uncertainty & Error Analysis 1 — Questions and Answers
Question 1: What does 'measurement uncertainty' represent in calibration?
- The definite error in a measurement result
- A parameter characterizing the dispersion of values that could reasonably be attributed to the measurand (Correct answer)
- The difference between the instrument reading and the true value
- The resolution limit of the measuring instrument
Correct answer: A parameter characterizing the dispersion of values that could reasonably be attributed to the measurand
Measurement uncertainty is a parameter that characterizes the range of values within which the true value is estimated to lie, reflecting the doubt about the measurement result.
Question 2: Type A evaluation of measurement uncertainty is based on:
- Manufacturer specifications and calibration certificates
- Expert judgment and historical data
- Statistical analysis of repeated measurement observations (Correct answer)
- Published reference tables and handbooks
Correct answer: Statistical analysis of repeated measurement observations
Type A uncertainty evaluation uses statistical methods applied to a series of repeated measurements to estimate the standard deviation and standard uncertainty.
Question 3: Type B evaluation of measurement uncertainty is characterized by:
- Repeated measurements and standard deviation calculations
- Information from sources other than repeated observations, such as certificates and specifications (Correct answer)
- Monte Carlo simulations only
- Regression analysis of calibration data
Correct answer: Information from sources other than repeated observations, such as certificates and specifications
Type B uncertainty evaluation uses information such as calibration certificates, manufacturer specifications, reference data, and expert judgment rather than statistical analysis of repeated measurements.
Question 4: The combined standard uncertainty (uc) is calculated from individual uncertainty components by:
- Summing all individual standard uncertainties directly
- Taking the arithmetic mean of all uncertainty contributions
- Taking the square root of the sum of the squares of individual standard uncertainties (Correct answer)
- Multiplying each uncertainty by its sensitivity coefficient and averaging
Correct answer: Taking the square root of the sum of the squares of individual standard uncertainties
Combined standard uncertainty is obtained by the root sum of squares (RSS) method, which combines individual uncertainty components assuming they are uncorrelated.
Question 5: A coverage factor of k=2 applied to the combined standard uncertainty typically corresponds to a confidence level of approximately:
- 68%
- 90%
- 95% (Correct answer)
- 99.7%
Correct answer: 95%
For a normal distribution, a coverage factor k=2 yields an expanded uncertainty that encompasses approximately 95% of the distribution of values.
Question 6: Which international document provides the primary guidance on expressing measurement uncertainty?
- ISO 9001
- JCGM 100:2008 (GUM — Guide to the Expression of Uncertainty in Measurement) (Correct answer)
- ANSI/NCSL Z540-1
- ASTM E177
Correct answer: JCGM 100:2008 (GUM — Guide to the Expression of Uncertainty in Measurement)
The GUM (JCGM 100:2008) is the internationally recognized guide that establishes general rules for evaluating and expressing measurement uncertainty.
Question 7: What is the key difference between accuracy and precision in measurement?
- Accuracy refers to repeatability; precision refers to closeness to the true value
- Accuracy describes closeness to the true value; precision describes the spread of repeated measurements (Correct answer)
- Accuracy and precision are interchangeable terms in metrology
- Accuracy applies to analog instruments; precision applies to digital instruments
Correct answer: Accuracy describes closeness to the true value; precision describes the spread of repeated measurements
Accuracy indicates how close a measurement is to the accepted true value, while precision describes the reproducibility or spread of repeated measurements regardless of how close they are to the true value.
CCT Measurement Uncertainty & Error Analysis 1
What does 'measurement uncertainty' represent in calibration?