Measurement Uncertainty
Probability and Statistics · FE Reference Handbook section
Handbook notes for this section
Definitions and conditions exactly as the handbook states them.
- Measurement uncertainty is defined as: A quantitative estimate of the range of values about the reported or measured value in
- which the true value is believed to lie. [Source: ISO JCGM 200:2012, definition 2.26]
- Given a desired state or measurement y, which is a function of different measured or available states xi:
- This represents 95% of the area under a Normal probability distribution and is often called 2 sigma.
Core formulas for this FE topic
Definitions, applicability, units, assumptions and worked examples for each relation.
Worked exam-style examples
The four ways this section is written on the real exam — thoughts first, then equations, then substitution.
A calibration lab combines two sources of measurement uncertainty. Given uncertainty of quantity 1 (u1) = 2.4000; uncertainty of quantity 2 (u2) = 0.7500, determine the combined uncertainty (uc).
Given
Find
combined uncertainty (uc)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except uc is given, so isolate uc symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for uc:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning uc = 2.5145 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 5.0289 — kept a factor of two that cancels in the correct rearrangement.
- 1.2572 — dropped that same factor in the other direction.
- 2.7659 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
An engineer reports the measurement uncertainty of an instrument reading. Given uncertainty of quantity 2 (u2) = 0.8500; combined uncertainty (uc) = 7.0500, determine the uncertainty of quantity 1 (u1).
Given
Find
uncertainty of quantity 1 (u1)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except u1 is given, so isolate u1 symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for u1:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning u1 = 6.9986 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 13.9971 — kept a factor of two that cancels in the correct rearrangement.
- 3.4993 — dropped that same factor in the other direction.
- 7.6984 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
The measurement uncertainty of a length gauge is propagated from two components. Given uncertainty of quantity 1 (u1) = 1.8500; combined uncertainty (uc) = 2.0900, determine the uncertainty of quantity 2 (u2).
Given
Find
uncertainty of quantity 2 (u2)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except u2 is given, so isolate u2 symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for u2:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning u2 = 0.9724 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 1.9448 — kept a factor of two that cancels in the correct rearrangement.
- 0.4862 — dropped that same factor in the other direction.
- 1.0697 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
A calibration lab combines two sources of measurement uncertainty. Given uncertainty of quantity 1 (u1) = 4.3000; uncertainty of quantity 2 (u2) = 2.6000, determine the combined uncertainty (uc).
Given
Find
combined uncertainty (uc)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except uc is given, so isolate uc symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for uc:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning uc = 5.0249 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 10.0499 — kept a factor of two that cancels in the correct rearrangement.
- 2.5125 — dropped that same factor in the other direction.
- 5.5274 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
An engineer reports the measurement uncertainty of an instrument reading. Given uncertainty of quantity 2 (u2) = 4.2000; combined uncertainty (uc) = 2.6800, determine the uncertainty of quantity 1 (u1).
Given
Find
uncertainty of quantity 1 (u1)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except u1 is given, so isolate u1 symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for u1:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning u1 = 0.0100 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.0200 — kept a factor of two that cancels in the correct rearrangement.
- 0.0050 — dropped that same factor in the other direction.
- 0.0110 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
The measurement uncertainty of a length gauge is propagated from two components. Given uncertainty of quantity 1 (u1) = 0.6500; combined uncertainty (uc) = 2.7600, determine the uncertainty of quantity 2 (u2).
Given
Find
uncertainty of quantity 2 (u2)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except u2 is given, so isolate u2 symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for u2:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning u2 = 2.6824 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 5.3647 — kept a factor of two that cancels in the correct rearrangement.
- 1.3412 — dropped that same factor in the other direction.
- 2.9506 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
A calibration lab combines two sources of measurement uncertainty. Given uncertainty of quantity 1 (u1) = 1.4500; uncertainty of quantity 2 (u2) = 1.3000, determine the combined uncertainty (uc).
Given
Find
combined uncertainty (uc)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except uc is given, so isolate uc symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for uc:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning uc = 1.9474 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 3.8949 — kept a factor of two that cancels in the correct rearrangement.
- 0.9737 — dropped that same factor in the other direction.
- 2.1422 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
An engineer reports the measurement uncertainty of an instrument reading. Given uncertainty of quantity 2 (u2) = 1.6500; combined uncertainty (uc) = 2.2000, determine the uncertainty of quantity 1 (u1).
Given
Find
uncertainty of quantity 1 (u1)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except u1 is given, so isolate u1 symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for u1:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning u1 = 1.4552 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 2.9103 — kept a factor of two that cancels in the correct rearrangement.
- 0.7276 — dropped that same factor in the other direction.
- 1.6007 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
The measurement uncertainty of a length gauge is propagated from two components. Given uncertainty of quantity 1 (u1) = 3.7000; combined uncertainty (uc) = 2.2300, determine the uncertainty of quantity 2 (u2).
Given
Find
uncertainty of quantity 2 (u2)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except u2 is given, so isolate u2 symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for u2:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning u2 = 0.0100 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.0200 — kept a factor of two that cancels in the correct rearrangement.
- 0.0050 — dropped that same factor in the other direction.
- 0.0110 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty
A calibration lab combines two sources of measurement uncertainty. Given uncertainty of quantity 1 (u1) = 2.9500; uncertainty of quantity 2 (u2) = 4.6500, determine the combined uncertainty (uc).
Given
Find
combined uncertainty (uc)
Start with the thinking
- The governing relation printed in this handbook section is Measurement uncertainty propagation.
- Everything except uc is given, so isolate uc symbolically first — never rearrange after the numbers are in.
- Tabulate each given with its unit and confirm the units are consistent with the relation before substituting.
- Measurement uncertainty from independent sources combines in quadrature to give the combined uncertainty.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for uc:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning uc = 5.5068 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 11.0136 — kept a factor of two that cancels in the correct rearrangement.
- 2.7534 — dropped that same factor in the other direction.
- 6.0575 — rounded an intermediate value before the final step.
Reference: FE Handbook — Measurement Uncertainty