This Exam P sample reference tests Expectation of a Function. This problem can be evaluated without copying the three candidate integrals. Symmetry gives the signed deviation, and a short negative-part correction gives an absolute expectation of 13/12; the matching expression is choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AIts final integral uses 2-x on the entire interval from 3 to 5, making that positive-tail contribution negative. The displayed expression evaluates to -7/12 rather than a nonnegative absolute expectation.
BIt uses 2-x throughout the first density branch from 1 to 3 and fails to reverse the sign at x=2. That expression evaluates to 2/3 instead of 13/12.
CIt reverses the correct absolute-value sign on every interval, producing the negative of the desired integral, -13/12.
DIt integrates x-2 over the full support without splitting at 2. This gives E[X-2]=1, the signed deviation rather than the absolute deviation.
Original practice · fully worked
Original variant: expected absolute sensor penalty
A sensor adjustment R takes values -2, 0, 3, and 7 with probabilities 0.15, 0.25, 0.40, and 0.20, respectively. A quality penalty is defined as C=|R-2|. Calculate E[C].
A 0.30
B 2.30
C 2.50
D 3.00
E 5.00
Variant answer in brief
The four penalties are 4, 2, 1, and 5. Weighting them by their probabilities gives 0.15(4)+0.25(2)+0.40(1)+0.20(5)=2.50, so choice C.
Setup
Setup
Evaluate the absolute penalty at each possible adjustment value.
C(−2)=4,C(0)=2,C(3)=1,C(7)=5
Model
Model
Use the expectation-of-a-function formula for a discrete distribution.
E[C]=r∑∣r−2∣Pr(R=r)
Compute
Compute
Multiply each penalty by its probability and add the four contributions.
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