This Exam P sample reference tests Binomial Distribution. The count is binomial with n=18 and p=0.15. Since (n+1)p=2.85 is not an integer, its unique mode is floor(2.85)=2, so choice C is correct.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis treats the low probability for one trial as implying a zero modal count and ignores the accumulation across eighteen trials.
BThis uses an incorrect mode rule such as floor(np)-1, moving one count below the actual peak.
DThis rounds the mean np=2.7 to 3; a binomial mode is determined by (n+1)p rather than ordinary rounding of the mean.
EThis moves farther into the decreasing upper side of the binomial mass after the peak near the mean.
Original practice · fully worked
Original variant: inferred inspection rate
The number X of flagged items in ten independent inspections is binomial with an unknown flag probability. The probability of exactly four flags is 1.5 times the probability of exactly three flags. Calculate the mode of X.
A 3
B 4
C 5
D 6
E 10
Variant answer in brief
The adjacent-mass ratio gives 7p/[4(1-p)]=1.5, so p=6/13. Then (10+1)p=66/13 lies between 5 and 6, making 5 the unique mode and choice C correct.
Setup
Setup
Use the ratio of adjacent binomial masses to recover the unknown probability.
Pr(X=3)Pr(X=4)=410−31−pp=1.5
Model
Model
Solve the ratio equation for p.
4(1−p)7p=23
p=136
Compute
Compute
Apply the binomial mode rule after inferring the parameter.
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