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Fingerprint Analysis & Documentation – Week 6 Lecture 1 Study Notes

Review of Week 5 and Recurring Problems

  • False‐positive fingerprint conclusions discussed last week (cases in Scotland & USA) serve as cautionary tales.
    • Core failures identified:
    • Incomplete analysis of the unknown (latent) print during the ACE (Analysis–Comparison–Evaluation) process.
    • Inadequate or missing documentation of each ACE step → makes peer review or court scrutiny impossible.
    • Verification phase of ACE‐V (the “V”) failed as a QA/QC back-stop: verifiers either rubber-stamped the call or were deprived of sufficient documentation to evaluate it.
    • Ethical implication: A deficient workflow can place innocent people at risk and erode public trust in forensic science.

Wrongful-Conviction Vocabulary

  • Faulty evidence
    • Testimony not actually supported by underlying science.
  • Misleading evidence
    • Opinions overstated or limitations/opposing data concealed.
  • Why it matters: Courts and juries often treat forensic testimony as authoritative; unqualified statements magnify the danger.

Illustrative Misidentification Case Files

  • Madrid Train Bombing (2004) – Brandon Mayfield mis-ID by FBI.
  • Boston example (slide shows but not detailed) – reminder that high-profile errors recur.
  • Elkhart, Indiana
    • Chart displayed latent vs. inked print; illustrates subjective mark-ups and potential quality pitfalls.
    • Raises question: “Chart Quality?”—underscores documentation standards.

Foundations of Fingerprint Identification (Key Scientific Assertions)

  • Individuality
    • No two fingerprints share identical ridge configurations (Level 1 patterns & Level 2 minutiae).
  • Permanence
    • Once formed in utero, ridge detail persists throughout life barring dermal damage.
    • Evidence base:
    • Welcker longitudinal study (1856–1897) → identical impressions ~41 yrs apart.
    • Faulds 1880; Galton 1892; Herschel 1916: early confirmations.
    • Modern confirmations: Okajima 1979; Wertheim et al 2002; Wan et al 2003.
    • Exceptions / modifiers:
    • Scarring penetrating dermis.
    • Aging → ridges flatten, dermis loses elasticity, pattern visibility lowers (Okajima 1979).
    • Certain systemic medications can erode ridge detail.
    • Human growth (child to adult) scales print but topology remains.
  • Recoverability
    • Latent ridge detail can be transferred to and subsequently visualized on diverse substrates.

Probability & Statistical Models for Identification

  • Approx. two dozen models proposed since 1892; none yet validated under operational casework conditions.
  • Common theme: all predict astronomically low probability that two random individuals share a given arrangement of minutiae—yet assumptions & independence questions remain.

Galton Model (1892)

  • Relied on predicting minutiae occurrence from surrounding ridge layout; lacked empirical frequency data.
  • Calculated joint probability of a specific minutiae arrangement as \tfrac{1}{68\,000\,000\,000}.
  • Crude by modern standards but seminal—it framed the individuality debate.

Henry Model (1900)

  • Treated each ridge characteristic (rc) as an independent, identically distributed (i.i.d.) event with p=\tfrac{1}{4}.
  • Probability of matching 12 rc: \left(\tfrac{1}{4}\right)^{12}=6\times10^{-8}\,\,(\text{≈ }1:!17\text{ million}).
  • Added pattern‐type weighting—equates to 2 extra rc → for a whorl w/12 rc:
    \left(\tfrac{1}{4}\right)^{14}=4\times10^{-9}\,\,(\text{≈ }1:!270\text{ million}).
  • Critique: rc independence assumption unrealistic; still influential—spawned numerical thresholds (e.g., “12‐point rule”).

Balthazard Numerical Standard (1911)

  • First explicit numeric threshold: ≥17 matching rc → identification.
  • Applied probabilistic reasoning to a world population of 1.5 billion.
  • Allowed reduced threshold for local‐suspect pools (town/country)—foreshadowed conditional probability/bayesian thinking.

Locard’s Tripartite Rule (1914)

  • NOT a statistical model—rather, an evidential sufficiency heuristic.
    1. >12 clear matching rc → certainty beyond debate.
    2. 8–12 rc → identification “marginal”; certainty depends on five quality factors:
      a) overall clarity; b) rarity of minutiae; c) clear core & delta; d) visible pores; e) agreement of ridge/furrow width, flow direction, bifurcation angles.
    3. <8 rc → cannot establish ID; only conveys proportional presumption.
  • Requires at least two competent examiners to concur for parts 1 & 2.
  • Modern view (Champod 1995) sees part 3 as proto‐probabilistic.

Synthesis of Models

  • Regardless of approach, calculated random match probabilities extremely small.
  • Limitation: none validated with large, real latent databases in operational environments → gap between theoretical certainty & courtroom reality.

Contemporary Fingerprint Research

Latent Residue: Drugs

  • Sensitive MS techniques detect cocaine, heroin, morphine down to tens of picograms ( 10^{-11}\,g ) from a single print.
  • Also identifies prescription medications—potential investigative intelligence (timeline, user habits).

Latent Residue: Aging of Prints

  • Mass‐spec imaging tracked triacylglycerol decay over 7 days for individual donors.
  • Rate of chemical degradation donor‐specific; still works on powder‐dusted prints.
  • Could eventually provide “time‐since‐deposition” estimates.

Processing New Polymer Banknotes

  • Pre‐2015 paper bills: ninhydrin / 1,2‐indanedione effective.
  • Post‐2015 polymer notes:
    1. Apply cyanoacrylate fuming (limited to transparent windows).
    2. For opaque inked areas use Vacuum Metal Deposition (VMD): vaporized gold adheres to fatty residues, zinc coats gold creating contrast.
  • Alternative research (Canadian study JEFSR 2016):
    • Natural‐IR powders + forensic light sources.
    • Silicone casting material & gelatin lifters as non-destructive options.

FRStat Software (Defense Forensic Science Center)

  • Provides likelihood ratios to quantify evidential strength.
  • Intended to complement examiner opinion, not replace it.
  • Caveats:
    • Quality of output proportional to quality of minutiae counts/classification fed into algorithm.
    • Must be internally validated before courtroom deployment.

Sex Determination via Amino Acid Profiling

  • Women’s latent sweat shows ≈2× higher concentrations of specific amino acids.
  • 2015 Analytical Chemistry study (Vol 87 Issue 22) demonstrates laboratory viability; field kit development ongoing.
  • Possible role as investigative adjunct or to corroborate other biological traces.

Vision for the Future – Insights from Champod (2015) Post-NAS Report

A) Clarifying the Inference Process

  • Advocate abandoning categorical “identification”/“individualization” claims.
  • Replace with Bayesian framework: evaluate P(E|Hp) vs. P(E|Hd) (evidence under prosecution vs. defence hypotheses).

B) Improving Transparency

  • Full disclosure of case notes, comparison rationale, and analytic uncertainties.
  • Move away from authority-based assertions (“unique because I say so”).

C) Avoiding the “Expert Black Box”

  • Distinguish individual examiner error rate from systemic error rate.
  • A claimed 0.1 % false positive rate may be misleadingly small unless accompanied by context (complexity, substrate, lab practices).

D) Introducing Statistics & Reconciling Conflicts

  • Probability statements inevitable; courts must prepare for likelihood ratios.
  • Research & operational validation still lacking → urgent priority.
  • Training programs will need overhaul; legal community education essential.

E) Quantifying Mark Quality

  • Need standardized metrics to grade latent print “information content.”
  • Allows triage: high‐quality marks → minimal bias safeguards; low‐quality → strict blind verification & context management.

F) Managing Bias

  • Bias‐mitigation proportional to evidentiary quality.
    • High clarity: lower risk but still benefits from safeguards.
    • Low clarity: enforce blinding, sequential unmasking, peer review.

Ethical, Legal & Practical Takeaways

  • Robust documentation and validated quantitative tools are central to sustaining credibility.
  • Courts increasingly expect empirical foundations; forensic discipline must evolve from tradition-based to evidence-based practice.
  • Interdisciplinary collaboration (statisticians, cognitive scientists, chemists, software engineers) will drive next‐gen fingerprint science.