Impression Evidence and Bloodstain Patterns (lecture 14)

Impression Evidence - Summary

Good Comparative Evidence

  • Criteria for Useful Comparisons
    • Variation: Determine how much variation is acceptable and detectable.
    • Scoreable/Classifiable: Evidence must be quantifiable.
    • Identification Stages: Importance of empirical observation and statistical approaches in forensic science.

Forensic Science Practices

  • Historical Shift: Move from traditional methods to empirical observations.
  • Validation Studies: Essential for establishing reliability and accuracy in forensic methods.

President’s Council of Advisors on Science and Technology (PCAST) 2016

  • Focus: Improving forensic methods since the 2009 NAS report.
  • Key Question: Are forensic analyses like DNA and fingerprints backed by reproducible research?

Types of Validity in Forensic Evidence

  • Foundational Validity: Must be based on repeatable and accurate studies.
  • Applied Validity: Must be reliably applied in practical scenarios.
  • Conclusion from PCAST: Bitemark analysis lacks reliable validity, resources should not be expended on it.

Challenges in Forensic Analysis

  • Difficulty in developing accurate measures and databases for characteristic frequency.
  • Changing materials complicate probability estimates.

Bloodstain Pattern Analysis (BSPA)

  • Definition: Analysis of blood to interpret events of bloodshed.
  • Importance: Understanding volume, shape, and distribution aids in investigation.

Physical Properties of Blood

  • Behavior: Blood behaves predictably under force, forming spheres due to surface tension.
  • Drying Process: Stains dry inward from the perimeter, influenced by volume and environmental conditions.

Analyzing Bloodstains

  • Shape and Directionality: Stain shape provides information about direction and impact.
  • Height Effect: Blood drops increase in diameter as height increases until terminal velocity.

Bloodstain Pattern Categories

  • Passive Stains: Result from stationary dripping or contact with objects.
  • Dynamic Patterns: Created by force, showing blood projection due to an action.
  • Drip Patterns: Indicate movement; pools suggest the direction and activity of individuals.
  • Contact Stains: Include wipes and swipes, useful in determining event sequences and directions.