pdf_signature_extraction/paper/paper_a_discussion_v3.md

V. Discussion

A. Non-Hand-Signing Detection as a Distinct Problem

Our results highlight the importance of distinguishing non-hand-signing detection from the well-studied signature forgery detection problem. In forgery detection, the challenge lies in modeling the variability of skilled forgers who produce plausible imitations of a target signature. In non-hand-signing detection the signer's identity is not in question; the challenge is distinguishing between legitimate intra-signer consistency (a CPA who signs similarly each time) and image-level reproduction of a stored signature (a CPA whose signature on each report is a byte-level or near-byte-level copy of a single source image).

This distinction has direct methodological consequences. Forgery detection systems optimize for inter-class discriminability---maximizing the gap between genuine and forged signatures. Non-hand-signing detection, by contrast, requires sensitivity to the upper tail of the intra-class similarity distribution, where the boundary between consistent handwriting and image reproduction becomes ambiguous. The dual-descriptor framework we propose---combining semantic-level features (cosine similarity) with structural-level features (dHash)---addresses this ambiguity in a way that single-descriptor approaches cannot.

B. Continuous-Quality Spectrum vs. Discrete-Behavior Regimes

The most consequential empirical finding of this study is the asymmetry between signature level and accountant level revealed by the three-method convergent analysis and the Hartigan dip test (Sections IV-D and IV-E).

At the per-signature level, the distribution of best-match cosine similarity is not cleanly bimodal. Firm A's signature-level cosine is formally unimodal (dip test p = 0.17) with a long left tail. The all-CPA signature-level cosine is multimodal (p < 0.001), but its structure is not well approximated by a two-component Beta mixture: BIC clearly prefers a three-component fit, and the 2-component Beta crossing and its logit-GMM counterpart disagree sharply on the candidate threshold (0.977 vs. 0.999 for Firm A). The BD/McCrary discontinuity test locates its transition at 0.985---inside the non-hand-signed mode rather than at a boundary. Taken together, these results indicate that non-hand-signed signatures form a continuous quality spectrum rather than a discrete class. Replication quality varies continuously with scan equipment, PDF compression, stamp pressure, and firm-level e-signing system generation, producing a heavy-tailed distribution that no two-mechanism mixture explains.

At the per-accountant aggregate level the picture reverses. The distribution of per-accountant mean cosine (and mean dHash) is unambiguously multimodal. A BIC-selected three-component Gaussian mixture cleanly separates (C1) a high-replication cluster dominated by Firm A, (C2) a middle band shared by the other Big-4 firms, and (C3) a hand-signed-tendency cluster dominated by smaller domestic firms. The two-component marginal crossings (cosine = 0.945, dHash = 8.10) are sharp and mutually consistent.

The substantive interpretation is simple: pixel-level output quality is continuous, but individual signing behavior is close to discrete. A given CPA tends to be either a consistent user of non-hand-signing or a consistent hand-signer; it is the mixing of these discrete behavioral types at the firm and population levels that produces the quality spectrum observed at the signature level. Methodologically, this implies that the three-method convergent framework (KDE antimode / BD-McCrary / Beta mixture) is meaningful at the level where the mixture structure is statistically supported---namely the accountant level---and serves as diagnostic evidence rather than a primary classification boundary at the signature level.

C. Firm A as a Replication-Dominated, Not Pure, Population

A recurring theme in prior work that treats Firm A or an analogous reference group as a calibration anchor is the implicit assumption that the anchor is a pure positive class. Our evidence across multiple analyses rules out that assumption for Firm A while affirming its utility as a calibration reference.

Three convergent strands of evidence support the replication-dominated framing. First, the interview evidence itself: Firm A partners report that most certifying partners at the firm use non-hand-signing, without excluding the possibility that a minority continue to hand-sign. Second, the statistical evidence: Firm A's per-signature cosine distribution is unimodal long-tail rather than a tight single peak; 92.5% of Firm A signatures exceed cosine 0.95, with the remaining 7.5% forming the left tail. Third, the accountant-level evidence: 32 of the 180 Firm A CPAs (18%) fall into the middle-band C2 cluster rather than the high-replication C1 cluster of the accountant GMM---consistent with the interview-acknowledged minority of hand-signers.

The replication-dominated framing is internally coherent with all three pieces of evidence, and it predicts and explains the residuals that a "near-universal" framing would be forced to treat as noise. We therefore recommend that future work building on this calibration strategy should explicitly distinguish replication-dominated from replication-pure calibration anchors.

D. The Style-Replication Gap

Within the 71,656 documents exceeding cosine 0.95, the dHash descriptor partitions them into three distinct populations: 29,529 (41.2%) with high-confidence structural evidence of non-hand-signing, 36,994 (51.7%) with moderate structural similarity, and 5,133 (7.2%) with no structural corroboration despite near-identical feature-level appearance. A cosine-only classifier would treat all 71,656 documents identically; the dual-descriptor framework separates them into populations with fundamentally different interpretations.

The 7.2% classified as "high style consistency" (cosine > 0.95 but dHash > 15) are particularly informative. Several plausible explanations may account for their high feature similarity without structural identity, though we lack direct evidence to confirm their relative contributions. Many accountants may develop highly consistent signing habits---using similar pen pressure, stroke order, and spatial layout---resulting in signatures that appear nearly identical at the semantic feature level while retaining the microscopic variations inherent to handwriting. Some may use signing pads or templates that further constrain variability without constituting image-level reproduction. The dual-descriptor framework correctly identifies these cases as distinct from non-hand-signed signatures by detecting the absence of structural-level convergence.

E. Value of a Replication-Dominated Calibration Group

The use of Firm A as a calibration reference addresses a fundamental challenge in document forensics: the scarcity of ground truth labels. In most forensic applications, establishing ground truth requires expensive manual verification or access to privileged information about document provenance. Our approach leverages domain knowledge---the established prevalence of non-hand-signing at a specific firm---to create a naturally occurring reference population within the dataset.

This calibration strategy has broader applicability beyond signature analysis. Any forensic detection system operating on real-world corpora can benefit from identifying subpopulations with known dominant characteristics (positive or negative) to anchor threshold selection, particularly when the distributions of interest are non-normal and non-parametric or mixture-based thresholds are preferred over parametric alternatives. The framing we adopt---replication-dominated rather than replication-pure---is an important refinement of this strategy: it prevents overclaim, accommodates interview-acknowledged heterogeneity, and yields classification rates that are internally consistent with the data.

F. Pixel-Identity as Annotation-Free Ground Truth

A further methodological contribution is the use of byte-level pixel identity as an annotation-free gold positive. Handwriting physics makes byte-identity impossible under independent signing events, so any pair of same-CPA signatures that are byte-identical after crop and normalization is an absolute positive for non-hand-signing, requiring no human review. In our corpus 310 signatures satisfied this condition, and all candidate thresholds (KDE crossover 0.837; accountant-crossing 0.945; canonical 0.95) classify this population perfectly. Paired with a conservative low-similarity anchor, this yields EER-style validation without the stratified manual annotation that would otherwise be the default methodological choice. We regard this as a reusable pattern for other document-forensics settings in which the target mechanism leaves a byte-level physical signature in the artifact itself.

G. Limitations

Several limitations should be acknowledged.

First, comprehensive per-document ground truth labels are not available. The pixel-identity anchor is a strict subset of the true non-hand-signing positives (only those whose nearest same-CPA match happens to be byte-identical), and the low-similarity anchor (n = 35) is small because intra-CPA pairs rarely fall below cosine 0.70. The reported FAR values should therefore be read as order-of-magnitude estimates rather than tight bounds, and a modest expansion of the negative anchor---for example by sampling inter-CPA pairs under explicit accountant-level blocking---would strengthen future work.

Second, the ResNet-50 feature extractor was used with pre-trained ImageNet weights without domain-specific fine-tuning. While our ablation study and prior literature [20]--[22] support the effectiveness of transferred ImageNet features for signature comparison, a signature-specific feature extractor could improve discriminative performance.

Third, the red stamp removal preprocessing uses simple HSV color-space filtering, which may introduce artifacts where handwritten strokes overlap with red seal impressions. In these overlap regions, blended pixels are replaced with white, potentially creating small gaps in the signature strokes that could reduce dHash similarity. This effect would bias classification toward false negatives rather than false positives, but the magnitude has not been quantified.

Fourth, scanning equipment, PDF generation software, and compression algorithms may have changed over the 10-year study period (2013--2023), potentially affecting similarity measurements. While cosine similarity and dHash are designed to be robust to such variations, longitudinal confounds cannot be entirely excluded, and we note that our accountant-level aggregates could mask within-accountant temporal transitions.

Fifth, the classification framework treats all signatures from a CPA as belonging to a single class, not accounting for potential changes in signing practice over time (e.g., a CPA who signed genuinely in early years but adopted non-hand-signing later). Extending the accountant-level analysis to auditor-year units is a natural next step.

Sixth, the BD/McCrary transition estimates fall inside rather than between modes for the per-signature cosine distribution, reflecting that this test is a diagnostic of local density-smoothness violation rather than of two-mechanism separation. This result is itself an informative diagnostic---consistent with the dip-test and Beta-mixture evidence that signature-level cosine is not cleanly bimodal---but it means BD/McCrary should be interpreted at the accountant level for threshold-setting purposes.

Finally, the legal and regulatory implications of our findings depend on jurisdictional definitions of "signature" and "signing." Whether non-hand-signing of a CPA's own stored signature constitutes a violation of signing requirements is a legal question that our technical analysis can inform but cannot resolve.