DAY 12 — Sources of Uncertainty

What are sources of measurement uncertainty?

Multiple input quantities and other factors that influence the measurement result

Does measurement uncertainty come from only one source?

No

At what stages can uncertainty enter the measurement process?

At different stages including the standard method instrument operator and measurement result

What can contribute uncertainty from a reference standard?

Calibration uncertainty stability or drift resolution assigned or reference value and traceability information

What can contribute uncertainty from the UUT or instrument?

Resolution repeatability indication variation instrument characteristics and corrections

What can contribute uncertainty from the measurement method?

Method limitations approximations corrections mathematical model and setup or configuration

What environmental factors can contribute to uncertainty?

Temperature humidity pressure vibration and electromagnetic effects

Should every environmental factor automatically be included?

Each factor should be considered according to whether it significantly influences the result

What is repeatability?

Variation when repeated measurements are made under defined repeatability conditions

What example repeatability readings are shown?

100.10 → 100.12 → 100.09 → 100.11 °C

How can observed repeatability variation be used?

It can be used to estimate a standard uncertainty component

What are operator or setup effects?

Effects such as measuring or parallax probe placement alignment loading connection and handling

How can limited resolution affect uncertainty?

Limited resolution can contribute to uncertainty

Is resolution the only possible contributor to instrument uncertainty?

No

What calibration data can contribute to uncertainty?

Previous calibration results calibration certificates corrections correction factors and historical data

What information can manufacturer specifications provide?

Accuracy or tolerance limits stability resolution temperature coefficients and other performance characteristics

Should manufacturer specifications simply be copied into an uncertainty budget?

No

What are examples of reference data or external inputs?

Certified reference materials reference tables handbook data calibration standards and external measurements

Are all input quantities independent?

No

What happens when input quantities are correlated?

Covariance terms may need to be considered

When is the simplified RSS approach applicable?

When the relevant inputs are uncorrelated

What is the basic uncertainty-budget process?

Identify contributors estimate input values and uncertainties assign probability distributions determine standard uncertainties apply sensitivity coefficients check correlation and combine to calculate combined uncertainty

What is the first step in building an uncertainty budget?

Identify contributors

What is the second step in building an uncertainty budget?

Estimate input values and uncertainties

What is the third step in building an uncertainty budget?

Assign probability distributions

What is the fourth step in building an uncertainty budget?

Determine standard uncertainties

What is the fifth step in building an uncertainty budget?

Apply sensitivity coefficients

What is the sixth step in building an uncertainty budget?

Check for correlation

What is the seventh step in building an uncertainty budget?

Combine the contributions to calculate combined uncertainty

What is an uncertainty-budget red flag?

A neat uncertainty budget does not automatically mean it is complete

What questions should be asked when reviewing an uncertainty budget?

Were all significant contributors identified

What is the calibrator's mindset for uncertainty sources?

Do not start with the uncertainty calculator

start with the measurement

What question should be asked before calculating uncertainty?

What can influence my result?

What is the main lesson of Day 12?

A complete uncertainty evaluation begins by identifying the factors that can influence the measurement result