This Exam P sample reference tests Density Normalization. This problem normalizes a linearly increasing density and then inverts its CDF. The resulting 80th percentile is √(3.2)=1.78885, which selects choice E.
These notes identify the calculation error associated with each wrong letter when that error is reproducible.
AThis takes 80% of a half-unit rather than inverting the cumulative distribution.
BThis is approximately √(0.8), omitting the support-scale factor of 2.
CThis is √(1.6), which results from using c=1 rather than the normalized coefficient 1/2.
DThis is 0.80 times the upper endpoint 2, treating the distribution as uniform despite its increasing density.
Original practice · fully worked
Original variant: percentile across a density breakpoint
A laboratory response score X has density c for 0<x<2, density 2c for 2≤x<5, and density zero elsewhere, where c is a positive constant. Calculate the 60th percentile of X.
A 2.000
B 3.000
C 3.400
D 4.400
E 4.800
Variant answer in brief
Normalization gives c=1/8, and the lower segment contains probability 0.25. The remaining 0.35 needed to reach 0.60 lies in the upper segment, giving q=3.4 and choice C.
Setup
Setup
Normalize the two constant-density segments.
1=2c+3(2c)=8c
c=81
Model
Model
Determine which segment contains the requested percentile.
F(2)=2c=41<0.60
Compute
Compute
Add the lower-segment mass to the probability accumulated in the upper segment.
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