Original practice · fully worked
Original variant: calibrate an exponential deductible
A repair cost follows an exponential distribution with mean 1000. A plan pays the amount of each repair cost above an ordinary deductible d, with no upper limit. Determine d if the expected plan payment per repair is 300.
- A
300 - B
700 - C
1204 - D
2303 - E
3333
Answer and calculation
For an exponential cost with mean 1000, the expected excess over d is 1000 exp(-d/1000). Equating this to 300 gives d=1203.97, choice C.
Setup
Setup
Use the exponential survival function to express the expected excess payment.
Pr(X>x)=e−x/1000 E[(X−d)+]=∫d∞Pr(X>x)dx Model
Model
Evaluate the tail integral and set it equal to the target payment.
E[(X−d)+]=1000e−d/1000 1000e−d/1000=300 Compute
Compute
Take logarithms and solve for the deductible.
e−d/1000=0.3 d=−1000log(0.3)=1203.9728 Answer
Answer
The required deductible rounds to 1204.
1204(C)