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Measurement Uncertainty

Probability and Statistics · FE Reference Handbook section

Probability and Statistics
3 formulas
10 exam-style examples
~51 min
All Probability and Statistics lectures

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.

Example 1
Measurement uncertainty propagation — solve for combined uncertainty — Measurement Uncertainty

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

  • uncertaintyofquantity1(u1)=2.4000uncertainty of quantity 1 (u_{1}) = 2.4000
  • uncertaintyofquantity2(u2)=0.7500uncertainty of quantity 2 (u_{2}) = 0.7500

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for uc:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=2.4000,uncertaintyofquantity2(u2)=0.7500List the givens: uncertainty of quantity 1 (u_{1}) = 2.4000, uncertainty of quantity 2 (u_{2}) = 0.7500
  4. Step 4 — Substitute the given values:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  5. Step 5 — Evaluate:

    uc=2.5145uc = 2.5145
  6. Step 6 — Check: returning uc = 2.5145 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
uc=2.5145uc = 2.5145

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

Example 2
Measurement uncertainty propagation — solve for uncertainty of quantity 1 — Measurement Uncertainty (2)

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

  • uncertaintyofquantity2(u2)=0.8500uncertainty of quantity 2 (u_{2}) = 0.8500
  • combineduncertainty(uc)=7.0500combined uncertainty (uc) = 7.0500

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for u1:

    u1=uc2−u22u_{1} = \sqrt{u_c^2-u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity2(u2)=0.8500,combineduncertainty(uc)=7.0500List the givens: uncertainty of quantity 2 (u_{2}) = 0.8500, combined uncertainty (uc) = 7.0500
  4. Step 4 — Substitute the given values:

    u1=uc2−u22u_{1} = \sqrt{u_c^2-u_2^2}
  5. Step 5 — Evaluate:

    u1=6.9986u_{1} = 6.9986
  6. Step 6 — Check: returning u1 = 6.9986 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
u1=6.9986u_{1} = 6.9986

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

Example 3
Measurement uncertainty propagation — solve for uncertainty of quantity 2 — Measurement Uncertainty (3)

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

  • uncertaintyofquantity1(u1)=1.8500uncertainty of quantity 1 (u_{1}) = 1.8500
  • combineduncertainty(uc)=2.0900combined uncertainty (uc) = 2.0900

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for u2:

    u2=uc2−u12u_{2} = \sqrt{u_c^2-u_1^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=1.8500,combineduncertainty(uc)=2.0900List the givens: uncertainty of quantity 1 (u_{1}) = 1.8500, combined uncertainty (uc) = 2.0900
  4. Step 4 — Substitute the given values:

    u2=uc2−u12u_{2} = \sqrt{u_c^2-u_1^2}
  5. Step 5 — Evaluate:

    u2=0.9724u_{2} = 0.9724
  6. Step 6 — Check: returning u2 = 0.9724 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
u2=0.9724u_{2} = 0.9724

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

Example 4
Measurement uncertainty propagation — solve for combined uncertainty (case 2) — Measurement Uncertainty (4)

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

  • uncertaintyofquantity1(u1)=4.3000uncertainty of quantity 1 (u_{1}) = 4.3000
  • uncertaintyofquantity2(u2)=2.6000uncertainty of quantity 2 (u_{2}) = 2.6000

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for uc:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=4.3000,uncertaintyofquantity2(u2)=2.6000List the givens: uncertainty of quantity 1 (u_{1}) = 4.3000, uncertainty of quantity 2 (u_{2}) = 2.6000
  4. Step 4 — Substitute the given values:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  5. Step 5 — Evaluate:

    uc=5.0249uc = 5.0249
  6. Step 6 — Check: returning uc = 5.0249 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
uc=5.0249uc = 5.0249

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

Example 5
Measurement uncertainty propagation — solve for uncertainty of quantity 1 (case 2) — Measurement Uncertainty (5)

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

  • uncertaintyofquantity2(u2)=4.2000uncertainty of quantity 2 (u_{2}) = 4.2000
  • combineduncertainty(uc)=2.6800combined uncertainty (uc) = 2.6800

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for u1:

    u1=uc2−u22u_{1} = \sqrt{u_c^2-u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity2(u2)=4.2000,combineduncertainty(uc)=2.6800List the givens: uncertainty of quantity 2 (u_{2}) = 4.2000, combined uncertainty (uc) = 2.6800
  4. Step 4 — Substitute the given values:

    u1=uc2−u22u_{1} = \sqrt{u_c^2-u_2^2}
  5. Step 5 — Evaluate:

    u1=0.0100u_{1} = 0.0100
  6. Step 6 — Check: returning u1 = 0.0100 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
u1=0.0100u_{1} = 0.0100

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

Example 6
Measurement uncertainty propagation — solve for uncertainty of quantity 2 (case 2) — Measurement Uncertainty (6)

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

  • uncertaintyofquantity1(u1)=0.6500uncertainty of quantity 1 (u_{1}) = 0.6500
  • combineduncertainty(uc)=2.7600combined uncertainty (uc) = 2.7600

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for u2:

    u2=uc2−u12u_{2} = \sqrt{u_c^2-u_1^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=0.6500,combineduncertainty(uc)=2.7600List the givens: uncertainty of quantity 1 (u_{1}) = 0.6500, combined uncertainty (uc) = 2.7600
  4. Step 4 — Substitute the given values:

    u2=uc2−u12u_{2} = \sqrt{u_c^2-u_1^2}
  5. Step 5 — Evaluate:

    u2=2.6824u_{2} = 2.6824
  6. Step 6 — Check: returning u2 = 2.6824 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
u2=2.6824u_{2} = 2.6824

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

Example 7
Measurement uncertainty propagation — solve for combined uncertainty (case 3) — Measurement Uncertainty (7)

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

  • uncertaintyofquantity1(u1)=1.4500uncertainty of quantity 1 (u_{1}) = 1.4500
  • uncertaintyofquantity2(u2)=1.3000uncertainty of quantity 2 (u_{2}) = 1.3000

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for uc:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=1.4500,uncertaintyofquantity2(u2)=1.3000List the givens: uncertainty of quantity 1 (u_{1}) = 1.4500, uncertainty of quantity 2 (u_{2}) = 1.3000
  4. Step 4 — Substitute the given values:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  5. Step 5 — Evaluate:

    uc=1.9474uc = 1.9474
  6. Step 6 — Check: returning uc = 1.9474 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
uc=1.9474uc = 1.9474

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

Example 8
Measurement uncertainty propagation — solve for uncertainty of quantity 1 (case 3) — Measurement Uncertainty (8)

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

  • uncertaintyofquantity2(u2)=1.6500uncertainty of quantity 2 (u_{2}) = 1.6500
  • combineduncertainty(uc)=2.2000combined uncertainty (uc) = 2.2000

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for u1:

    u1=uc2−u22u_{1} = \sqrt{u_c^2-u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity2(u2)=1.6500,combineduncertainty(uc)=2.2000List the givens: uncertainty of quantity 2 (u_{2}) = 1.6500, combined uncertainty (uc) = 2.2000
  4. Step 4 — Substitute the given values:

    u1=uc2−u22u_{1} = \sqrt{u_c^2-u_2^2}
  5. Step 5 — Evaluate:

    u1=1.4552u_{1} = 1.4552
  6. Step 6 — Check: returning u1 = 1.4552 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
u1=1.4552u_{1} = 1.4552

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

Example 9
Measurement uncertainty propagation — solve for uncertainty of quantity 2 (case 3) — Measurement Uncertainty (9)

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

  • uncertaintyofquantity1(u1)=3.7000uncertainty of quantity 1 (u_{1}) = 3.7000
  • combineduncertainty(uc)=2.2300combined uncertainty (uc) = 2.2300

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for u2:

    u2=uc2−u12u_{2} = \sqrt{u_c^2-u_1^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=3.7000,combineduncertainty(uc)=2.2300List the givens: uncertainty of quantity 1 (u_{1}) = 3.7000, combined uncertainty (uc) = 2.2300
  4. Step 4 — Substitute the given values:

    u2=uc2−u12u_{2} = \sqrt{u_c^2-u_1^2}
  5. Step 5 — Evaluate:

    u2=0.0100u_{2} = 0.0100
  6. Step 6 — Check: returning u2 = 0.0100 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
u2=0.0100u_{2} = 0.0100

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

Example 10
Measurement uncertainty propagation — solve for combined uncertainty (case 4) — Measurement Uncertainty (10)

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

  • uncertaintyofquantity1(u1)=2.9500uncertainty of quantity 1 (u_{1}) = 2.9500
  • uncertaintyofquantity2(u2)=4.6500uncertainty of quantity 2 (u_{2}) = 4.6500

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

  1. Step 1 — State the governing relation:

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}
  2. Step 2 — Rearrange symbolically for uc:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  3. Step 3

    Listthegivens:uncertaintyofquantity1(u1)=2.9500,uncertaintyofquantity2(u2)=4.6500List the givens: uncertainty of quantity 1 (u_{1}) = 2.9500, uncertainty of quantity 2 (u_{2}) = 4.6500
  4. Step 4 — Substitute the given values:

    uc=u12+u22uc = \sqrt{u_1^2+u_2^2}
  5. Step 5 — Evaluate:

    uc=5.5068uc = 5.5068
  6. Step 6 — Check: returning uc = 5.5068 to

    uc=u12+u22u_c = \sqrt{u_1^2 + u_2^2}

    reproduces the given quantities, and both sides carry the same units.

Answer:
uc=5.5068uc = 5.5068

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

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