Module 2: The Nature of Physical Evidence - Part 1
Evidence in Crime Scene Investigation
Overview of Evidence
Focus on physical evidence and its characteristics.
Importance of careful analysis and understanding of evidence.
Physical Evidence vs. Testimonial Evidence
Physical evidence has greater power than testimonial evidence in defining crime scene events.
Physical evidence is tangible and harder to dispute.
Testimonial evidence can be influenced by fallacies and personal agendas, leading to misinterpretations.
Need for corroboration of testimonial evidence through objective means like crime scene analysis.
The relationship between physical evidence recovery and the development of an objective theory of the crime.
Importance of Crime Scene Awareness
Investigators must know what they are searching for and recognize relevant evidence.
Understanding what happened at the scene aids in identifying evidence.
Historical ease of evidence identification (fingerprints, blood stains).
Advancements in forensic technology making evidence identification more complex.
Example: TouchDNA can yield specific investigative information from minimal evidence.
Role of Investigators and Forensic Experts
No need to be an expert in every forensic discipline, but a working knowledge of physical evidence is essential.
Importance of effective evidence collection to avoid overlooking critical information.
Collaboration between field investigators and laboratory scientists is crucial.
Metaphor: "lab rats and field mice" must understand each other's roles.
Classification of Physical Evidence
Not all evidence provides equal value; can be categorized based on specificity.
Class Characteristics
Defined as characteristics common to several objects.
Serve forensics by excluding possibilities based on comparison with known items.
Examples include size, color, manufacturing patterns, and taxonomic classifications.
Example: Comparing a piece of glass from clothing to a known glass sample regarding color and refractive index.
If discrepancies exist, items can be concluded as not originating from the same source.
Matching characteristics do not confirm common origin but prevent exclusion based on absence of common characteristics.
Utility in court: often used as a screening mechanism to avoid unnecessary detailed examinations.
Antigen screening in blood samples serves as a practical example of class characteristics.
Individual Characteristics
Allow comparison to a specific object or individual, enabling inclusion or exclusion from being the source.
Defined as unique characteristics resulting from natural variations, damage, or wear.
Example: Fingerprints are unique; no two fingerprints are alike.
Fingerprint analysis results in either inclusion or exclusion results, but may also result in inconclusive outcomes due to insufficient detail.
Individualization is the primary goal for forensic scientists, providing compelling evidence of origin.
The Role of Context
Context is critical for the value of evidence; evidence without context holds no significance.
Example: DNA as a "golden standard"—presence does not equate to proof of a crime without context.
Totality of circumstances: all evidence should be considered together for effective investigation resolution.
Review of Class and Individual Characteristics
Class characteristics: associated only with a group, not a single source.
Individual characteristics: attributed to a particular source with a high degree of certainty.
The need for individual characteristics is emphasized, although they are rarely found.
Reminder: Always maintain context for better investigative clarity and understanding.