Original practice · fully worked
Original variant: conditional mean of a tiered review score
A quality count N has probabilities 0.50, 0.25, 0.15, 0.07, and 0.03 at N=0,1,2,3,4, respectively. A review score is 0 for N at most 1, 10 for N=2, 30 for N=3, and 60 for N=4. Given that the score is at least 30, calculate its conditional mean.
- A
5.4 - B
30.0 - C
39.0 - D
45.0 - E
60.0
Answer and calculation
A score of at least 30 restricts the count to three or four, with total probability 0.10. Weighting scores 30 and 60 by masses 0.07 and 0.03 gives conditional mean 39, choice C.
Setup
Setup
Identify the two count values admitted by the score condition.
{S≥30}={N=3}∪{N=4} Pr(S≥30)=0.07+0.03=0.10 Model
Model
Form the conditional score expectation from the two retained masses.
E[S∣S≥30]=0.1030(0.07)+60(0.03) Compute
Compute
Evaluate the weighted numerator and normalize.
30(0.07)+60(0.03)=3.90 E[S∣S≥30]=0.103.90=39.0 Answer
Answer
The conditional mean score is 39.
39.0(C)