Expected Values
Probability and Statistics · FE Reference Handbook section
Handbook notes for this section
Definitions and conditions exactly as the handbook states them.
- Let X be a discrete random variable having a probability
- Engineering Probability and Statistics
- The expected value of X is defined as
- The mean or expected value of the random variable X is now defined as
- while the variance is given by
- The standard deviation is given by
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.
An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 8.0000; probability 1 (p1) = 0.4800; outcome 2 (x2) = 14.5000, determine the expected value (EX).
Given
Find
expected value (EX)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except EX is given, so isolate EX 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for EX:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning EX = 11.3800 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 22.7600 — kept a factor of two that cancels in the correct rearrangement.
- 5.6900 — dropped that same factor in the other direction.
- 12.5180 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
A student finds the expected value of a two-outcome gamble. Given outcome 1 (x1) = 4.0000; probability 1 (p1) = 0.5000; expected value (EX) = 4.8800, determine the outcome 2 (x2).
Given
Find
outcome 2 (x2)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except x2 is given, so isolate x2 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x2:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x2 = 5.7600 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 11.5200 — kept a factor of two that cancels in the correct rearrangement.
- 2.8800 — dropped that same factor in the other direction.
- 6.3360 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
The expected value of a production yield is estimated from probabilities. Given outcome 1 (x1) = 5.0000; outcome 2 (x2) = 19.0000; expected value (EX) = 8.9600, determine the probability 1 (p1).
Given
Find
probability 1 (p1)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except p1 is given, so isolate p1 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for p1:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning p1 = 0.7171 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 1.4343 — kept a factor of two that cancels in the correct rearrangement.
- 0.3586 — dropped that same factor in the other direction.
- 0.7889 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 7.0000; probability 1 (p1) = 0.3800; outcome 2 (x2) = 11.0000, determine the expected value (EX).
Given
Find
expected value (EX)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except EX is given, so isolate EX 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for EX:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning EX = 9.4800 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 18.9600 — kept a factor of two that cancels in the correct rearrangement.
- 4.7400 — dropped that same factor in the other direction.
- 10.4280 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
A student finds the expected value of a two-outcome gamble. Given outcome 1 (x1) = 10.0000; probability 1 (p1) = 0.4000; expected value (EX) = 8.3500, determine the outcome 2 (x2).
Given
Find
outcome 2 (x2)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except x2 is given, so isolate x2 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x2:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x2 = 7.2500 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 14.5000 — kept a factor of two that cancels in the correct rearrangement.
- 3.6250 — dropped that same factor in the other direction.
- 7.9750 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
The expected value of a production yield is estimated from probabilities. Given outcome 1 (x1) = 5.0000; outcome 2 (x2) = 17.5000; expected value (EX) = 2.8600, determine the probability 1 (p1).
Given
Find
probability 1 (p1)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except p1 is given, so isolate p1 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for p1:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning p1 = 1.1712 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 2.3424 — kept a factor of two that cancels in the correct rearrangement.
- 0.5856 — dropped that same factor in the other direction.
- 1.2883 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 2.5000; probability 1 (p1) = 0.5500; outcome 2 (x2) = 12.0000, determine the expected value (EX).
Given
Find
expected value (EX)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except EX is given, so isolate EX 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for EX:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning EX = 6.7750 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 13.5500 — kept a factor of two that cancels in the correct rearrangement.
- 3.3875 — dropped that same factor in the other direction.
- 7.4525 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
A student finds the expected value of a two-outcome gamble. Given outcome 1 (x1) = 1.5000; probability 1 (p1) = 0.3000; expected value (EX) = 9.4800, determine the outcome 2 (x2).
Given
Find
outcome 2 (x2)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except x2 is given, so isolate x2 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for x2:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning x2 = 12.9000 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 25.8000 — kept a factor of two that cancels in the correct rearrangement.
- 6.4500 — dropped that same factor in the other direction.
- 14.1900 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
The expected value of a production yield is estimated from probabilities. Given outcome 1 (x1) = 2.0000; outcome 2 (x2) = 20.0000; expected value (EX) = 12.0500, determine the probability 1 (p1).
Given
Find
probability 1 (p1)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except p1 is given, so isolate p1 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for p1:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning p1 = 0.4417 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 0.8833 — kept a factor of two that cancels in the correct rearrangement.
- 0.2208 — dropped that same factor in the other direction.
- 0.4858 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values
An engineer computes the expected value of a warranty payout distribution. Given outcome 1 (x1) = 7.5000; probability 1 (p1) = 0.3800; outcome 2 (x2) = 18.5000, determine the expected value (EX).
Given
Find
expected value (EX)
Start with the thinking
- The governing relation printed in this handbook section is Expected value of a discrete random variable.
- Everything except EX is given, so isolate EX 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.
- The expected value of a discrete random variable weights each outcome by its probability.
Step-by-step solution
Step 1 — State the governing relation:
Step 2 — Rearrange symbolically for EX:
Step 3
Step 4 — Substitute the given values:
Step 5 — Evaluate:
Step 6 — Check: returning EX = 14.3200 to
reproduces the given quantities, and both sides carry the same units.
Why the other options are there
- 28.6400 — kept a factor of two that cancels in the correct rearrangement.
- 7.1600 — dropped that same factor in the other direction.
- 15.7520 — rounded an intermediate value before the final step.
Reference: FE Handbook — Expected Values