Module 2: The Nature of Physical Evidence - Part 2

Nature of Physical Evidence

• Class and Individual Characteristics

  • Definition: Class characteristics cannot be associated with a unique source while individual characteristics can.

• Product Rule

  • Definition: The product rule states that to determine the overall frequency of occurrence of a combination of characteristics, one must first determine the probability of each characteristic occurring separately and independently.

  • Formula: If characteristics A, B, and C occur independently, then frequency = P(A) \times P(B) \times P(C).

• Example of Blood Evidence

  • Class characteristic of blood: Blood can be classified into types (A, B, AB, O).

  • In the context of the O.J. Simpson case, two blood specimens identified as human type A.

  • Frequency of type A blood in the US population: Approximately 26%.

  • Conclusion: While class characteristics are insufficient for establishing a common origin, comparing additional blood factors increases the probability of a common source.

  • Specific finding: A blood sample with certain factors matched, yielding a probability of occurrence of about 0.44 (or 1 in 200 people).

  • Victims Nicole Brown Simpson and Ron Goldman did not match the blood factors.

Important Concepts Related to Physical Evidence

• Locard's Exchange Principle

  • Introduced by Dr. Edmund Locard, known as the father of modern crime labs.

  • Principle definition: "Every contact leaves its trace." This means that whenever two objects come into contact, material is exchanged between them.

  • Importance: This principle underpins trace evidence examination and informs crime scene analysis.

  • Application: The ability to obtain DNA from small traces like touch DNA supports this principle.

  • Caveat: Not all contacts will allow for reliable associations between the items; traces can disappear over time.

  • Historical note: The original phrase was about violent crimes, stating, "Every violent crime cannot occur without leaving a trace."

• Cross Contamination

  • Definition: When evidence is unintentionally contaminated due to improper handling or environmental factors (e.g., sneezing).

  • Significance: A major concern in forensic investigations, especially for trace and serology evidence.

  • Example: The phantom of Heilbronn case illustrates severe problems due to contamination.

  • Preventative measures: It's crucial for investigators to isolate evidence and use validated collection techniques to reduce cross-contamination risks.

• Mechanical Fit (Fracture Matching)

  • Definition: A forensic process whereby broken pieces from a crime scene can be matched to a source item.

  • Significance: Establishing a mechanical fit is rare but can uniquely identify an item from others.

  • Unique formation ensures random breaks can often lead to specific identification.

• Limitations in Forensic Science

  • Difficulty in assigning exact probabilities to majority of class physical evidence, such as fibers or hairs, due to lack of comparative databases.

  • Importance of creating and updating statistical databases for evaluating significance of class evidence.

Challenges in Class and Individual Evidence

• Class Evidence

  • Value: While not unique, class evidence strengthens cases by aligning evidence with collective data from multiple factors.

  • Utilization: Used alongside personal experience for interpretation, especially in blood factors.

  • Comparison Example: Colored fibers; identification extends beyond just color to include chemical and physical properties.

• Individual Evidence

  • Challenge: Determining how many characteristics are necessary for an item to be individualized (e.g., bullet striations, fingerprint ridges).

  • Variability: Each collected physical evidence item's quality, composition, case history, and the examiner's experience determine its significance.

• Importance of Physical Evidence in Trials

  • Physical evidence holds significant weight in jury deliberations and can influence perceptions of guilt or innocence.

  • Requirement for relevance: Scientific evidence must be relevant to be admitted, ensuring it does not unfairly prejudice either side.

• Challenges in Comparison

  • Distinguishing characteristics between class and individual evidence can be difficult and subjective.

  • Example concerns: How many similarities are needed in a bullet mark to attribute it definitively to one weapon?

  • Practicality limits: Advances in analytical techniques also mean defining natural variation limitations is crucial to interpret data effectively.

Conclusion

• Final considerations: The understanding and application of these principles and evidence types are critical to forensic investigation.

• Imperative for thorough evidence collection and scientific evaluations to avoid prejudicing cases and ensure accurate democratic legal processes.