This Exam P sample reference tests Exponential Distribution. This is the expected excess of an exponential loss above a deductible. The probability of reaching the deductible is exp(−3/10), and memorylessness leaves mean excess 10, so expected payment is 10 exp(−0.3)≈7.41 and choice D.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThe value 2.59 is 10(1-exp(−0.3)), the expected amount of loss limited to the deductible. It is the retained layer, not the insurer's excess payment.
BThe value 5.19 is approximately twice the limited-loss amount 10(1-exp(−0.3)), double-counting a component that is not the claim payment.
CThe value 7.00 subtracts the deductible from the unconditional mean, 10-3, and ignores the zero payment when the loss does not exceed three.
EThe value 9.63 is E[X 1{X>3}]=13 exp(−0.3), which includes the full loss whenever a claim occurs instead of removing the deductible.
Original practice · fully worked
Original variant: repair payment under a severity mixture
A repair is minor with probability 0.30 and major with probability 0.70. Conditional on a minor repair, loss X is exponential with mean 4 units; conditional on a major repair, X is exponential with mean 10 units. A service plan pays the amount above a deductible of 5 units. Calculate the expected plan payment.
A 0.344
B 4.246
C 4.457
D 4.590
E 8.200
Variant answer in brief
For an exponential loss with mean μ, expected payment above five is μe to the power (−5/μ). Weighting the minor and major values by 0.30 and 0.70 gives 0.30(4 exp(−5/4))+0.70(10 exp(−1/2))≈4.590 and choice D.
Setup
Setup
Use the exponential stop-loss expectation separately within each repair class.
E[(X−5)+∣M]=4e−5/4
E[(X−5)+∣J]=10e−5/10
Model
Model
Average the two conditional payment means with their repair-class probabilities.
E[Y]=0.30(4e−5/4)+0.70(10e−1/2)
Compute
Compute
Evaluate the minor and major contributions and add them.
0.30(4e−5/4)=0.3438057562
0.70(10e−1/2)=4.245714618
E[Y]=4.589520374
Answer
Answer
The expected service-plan payment is approximately 4.590.
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