Phase 1.8 follow-up. Validates the v4.0 classifier family against
the only hard ground truth in the corpus: pixel_identical_to_closest=1
(byte-identical to nearest same-CPA neighbor; mathematically impossible
under independent hand-signing).
n = 262 pixel-identical Big-4 signatures.
Firm A 145
KPMG 8
PwC 107
EY 2
FAR (lower better; Wilson 95% CI for the misclassification rate):
PaperA box rule 0.00% [0.00%, 1.45%]
K=3 per-CPA hard label 0.00% [0.00%, 1.45%]
Reverse-anchor (calibr.) 0.00% [0.00%, 1.45%]
Per-firm: 0% misclass on every firm.
Reverse-anchor cut chosen by prevalence calibration (overall
replicated rate matches Paper A's 49.58%). Documented v4.0
limitation: no signature-level ground truth for hand-leaning
class, so cannot ROC-optimize the cut directly.
PwC's 107 pixel-identical signatures despite being the most
hand-leaning firm overall (Script 38 per-CPA P_C1=0.31)
illustrates the within-firm heterogeneity that v4.0's K=3
mixture captures: a PwC CPA can be hand-leaning on average
while still occasionally reusing template signatures.
Implication: at the only hard ground truth available in the
corpus, all three v4.0 classifiers achieve perfect detection.
This satisfies REQ-001 acceptance for pixel-identity FAR.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
