Forensic Science - Chapter 3:)

Any object that can: - Show a crime has (or has not) been committed. - Provide a link between a crime and its victim or its perpetrator.

• If investigations fail to recognize or properly preserve evidence (no amount of lab expertise or advanced technology can recover the lost value) • Proper collection and preservation are essential.

• Impossible to list all possible types of physical evidence. • Almost anything can serve as physical evidence depending on the case. • While no complete list exists, forensic science often discusses common categories (e.g., blood, hair, fibers, fingerprints, weapons).

• Recognizing evidence - allows it to be collected and analyzed. • The weight (importance or influence) of a piece of evidence: • Cannot be known in advance and ultimately decided by the jury of court.

• Blood, semen, and saliva • Documents • Drugs • Explosives • Fibers • Fingerprints • Firearms & ammunition • Glass • Hair • Impressions

• Organs & physiological fluids • Paint • Petroleum products • Plastic bags • Plastic, rubber, and other polymers • Powder residues • Soil and minerals • Tool marks • Vehicle lights • Wood & other vegetative matter

Forensic scientists examine physical evidence for 2 main purposes: 1. Identification 2. Comparision

• Purpose: Determine the physical or chemical identity of a substance. • Must be dine with as close to absolute certainty as current analytical techniques allow. • Ex. Identifying a white powder as cocaine, flour, or another substance.

• Process: Compare a suspect specimen (evidence from crime scene or suspect). • With a standard and reference specimen (known sample) • Goal: Determine if they have a common origin (i.e., if they match). • Ex. Comparing glass fragments from a suspect’s clothing to broken glass from a crime scene window).

• Adopt Standard Testing Procedures • Record and Reuse Results • Exclude All Other Substances

• Must use reliable tests that give characteristics results for specific standard materials. • Ensures consistency and accuracy across cases.

• Once test results are established, they are permanently recorded. • These results can be used repeatedly to confirm the identity of future suspect materials.

• Enough tests must be performed to eliminate every possible alternative. • Ensures identification is specific and not mistaken for another material.

• Crime laboratories are often asked to identify a wide range of materials. • Identification can involve both chemical composition and biological origin.

• Illicit Drugs - Determine the chemical composition of controlled substances (ex. confirming if a powder is cocaine, heroin, or another drug). • Fire Debris - Identify gasoline or other accelerants in residues from a fire scene. • Explosives - Determine the nature of explosive residues (ex. dynamite, TNT, or other bomb materials). • Biological Evidence - Identify blood, semen, or hair. - Routine tests also determine species origins (human vs. animal). • Other Organic Evidence - Identify wood or vegetative matter when relevant to the case.

• Comparison means checking if the evidence from a crime scene (suspect specimen) matches a known sample (standard/reference specimen) to see if they come from the same source (common origin).

• Testing: Both specimens undergo the same tests to ensure consistency. • Two-Step Forensic Comparison: - Step 1: Choose specific properties from both specimens to compare. - Step 2: After examination, the forensic scientists render a conclusion regarding the specimens’ origins.

• Comparison depends on objective testing and careful selection of properties. • The final conclusion must be based on the analysis of the observed similarities or differences.

• Helps evaluate the evidential value of a comparison between specimens. • Assists in determining whether specimens have a common origin.

Probability: The frequency of occurrence of an event.

• Simple case: Flipping a coin - probability is easy to calculate. • Complex Forensic Analysis: Exact probability is often impossible to define. • Probability helps scientists estimate how likely evidence comes from a certain source, even without exact calculations.

• Evidence that can be associated with a single source with an extremely high probability is said to have individual characteristics.

• Exact probability of a common origin cannot be mathematically stated. • The probability is so high it defies calculation or human comprehension.

• Fingerprints: Matching ridge patterns. • Bullets & Tool Marks: Random striation markings. • Tire or Footwear Impressions: Irregular and random wear patterns. • Handwriting: Unique writing characteristics. • Broken Objects: Irregular edges that fit together like a jigsaw puzzle. • Plastic Bags: Sequential striation marks across bags.