This Exam P sample reference tests Bayes' Theorem. This is an inverse Bayes calculation for an unknown within-group rate. If x is the female smoking rate, then the female and male smoker contributions are 0.4x and 0.12; requiring women to be 20% of all smokers gives x=0.075, or 7.50%, and choice A.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
BThe value 8.00% is 0.40(0.20), multiplying the female population share by the posterior female share among smokers. Those probabilities have incompatible conditioning.
CThe value 12.00% is the joint male-smoker probability 0.60(0.20), not the smoking rate within the female group.
DThe value 13.33% divides the erroneous 8% product by the male share 60%, compounding the conditioning error.
EThe value 20.00% simply repeats either the male smoking rate or the female share among smokers. Neither equals the requested female smoking rate.
Original practice · fully worked
Original variant: job mode behind a sampled audit record
A workflow processes 40% of jobs in standard mode, 35% in advanced mode, and 25% in diagnostic mode. A standard job produces one audit record, an advanced job produces two, and a diagnostic job produces four. One audit record is selected uniformly from all records. Calculate the probability that its job used diagnostic mode.
A 0.250
B 0.400
C 0.476
D 0.571
E 1.000
Variant answer in brief
Record sampling weights each job mode by its number of records. The standard, advanced, and diagnostic contributions are 0.40, 0.70, and 1.00 records per job, so diagnostic mode supplies 1/2.1=10/21≈0.476 of all records and choice C.
Setup
Setup
Weight each job-mode frequency by the number of records that one job in that mode creates.
wS=0.40(1)=0.40
wA=0.35(2)=0.70
wD=0.25(4)=1.00
Model
Model
A uniformly sampled record comes from a mode in proportion to that mode's record contribution.
Pr(D∣sampled record)=wS+wA+wDwD
Compute
Compute
Normalize the diagnostic contribution by the total number of records generated per job.
0.40+0.70+1.001.00=2.101=2110=0.4761904762
Answer
Answer
The sampled record has diagnostic-mode origin with probability approximately 0.476.
The 3108-page Probability Proof Manual reorganizes 718 verified Exam P solutions by syllabus skill and adds formula proofs, error patterns, and original worked practice.