Differentiating Class and Individual Evidence

Overview

• Author: Douglas P. Lyle
• Source: Article on differentiating class and individual evidence from "Forensics For Dummies."
• Update: March 26, 2016

Key Concepts

Class Characteristics

• Definition: Attributes that are shared by a group of objects.
• Example: Type of handgun (.38 caliber) used in a crime.
  • Every .38 caliber handgun can potentially be the murder weapon.

Individual Characteristics

• Definition: Features that can be traced back to a single, specific source.
• Example: Fingerprints on a handgun.
  • Unique fingerprint can link the gun to a specific person.

Evidence Types and Examples

• Blood Type:
  • Class characteristic: Type B blood can be shared by millions.
  • Individual characteristic: DNA testing can confirm a match to a specific individual.
• Case Study: Wayne Williams and the Atlanta Child Murders
  • Convicted based on class evidence: Fiber analysis.
  • 28 different fibers matched those from suspects’ belongings.
  • Demonstrated that collecting multiple class evidence increases the likelihood of conviction.

Individualizing Evidence

• Fingerprints and DNA:
  • Considered almost unique to individuals.
• Impression Evidence:
  • Includes bullet markings, shoeprints, tire tracks, and tool marks.
  • Unique patterns can link evidence to a specific source.
• Fracture or Tear Patterns:
  • Unique ways objects break (e.g., glass, paper, cloth).
  • Example: Matching glass pieces from a crime scene to a suspect's car.

Analytical Principle

• Uniqueness:
  • No two objects are exactly the same.
  • Applies to guns, glass fractures, and wear patterns on shoes/tires.

Summary

• Class evidence can be used to rule out suspects but rarely to convict.
• A large collection of class evidence can significantly strengthen a case.
• Individual characteristics provide conclusive links to specific sources.