Notes on Evidence Collection, Locard's Exchange Principle, and Evidence Classification

Locard's Exchange Principle

• Edmond Locard ($1877-1966$) studied crimes and established the first police crime lab in France.

• His theory states that contact between two entities (a victim and a suspect) will result in a mutual exchange of materials.

• These materials can be identified and analyzed to link the victim to the suspect.

• The principle implies that some trace interchange occurs whenever there is contact at a crime scene, forming a basis for linking people, places, and objects through physical evidence.

Four-way Linkage Theory

• The four-way linkage theory explains the relationships between the victim, suspect, crime scene, and physical evidence.

• Understanding these "links" will guide investigators in determining where evidence may be located and what evidence may still need to be identified.

• Links help establish the path of investigation: what to search for, where to search, and how items may connect to the involved parties and locale.

Identifying Evidence

• Investigators must determine whether a piece of evidence is valuable to the investigation.

• Key questions to assess value:

  • Is this unique?

  • Is there a low probability that this occurred by chance?

  • Is this inconsistent with the setting or character of the victim?

  • Could this be used to physically link the victim to the suspect or the suspect to the scene?

What Evidence Should Be Collected?

• Collect Everything

  • Investigators may be tempted to collect everything that could possibly be construed as evidence.

  • This approach will overwhelm the forensic facilities and may result in false leads, which will waste time and resources.

• Probative Value Only

  • Investigators may focus only on evidence that has probative value, or evidence that proves or may be used to prove a fact.

  • This may result in key evidence being overlooked.

• Knowing What's Evidence Takes Time and Skill

  • Sorting through items and objects to determine potential relevance is a skill mastered through substantial training and experience.

Physical Evidence

• Not all types of physical evidence will be able to directly link to or identify a suspect.

• In fact, the most common use of physical evidence found at crime scenes is to identify an unknown substance.

  • Examples include the identification of suspected controlled substances or poisons, identification of accelerants in fire debris, blood alcohol levels, and toxicology of blood specimens (Lee and Pagliaro).

• Moreover, physical evidence can provide indirect investigative information or leads to solve a crime.

  • This is the most important and significant use of physical evidence collected from the crime scene.

• Not every crime scene will have a 'smoking-gun' or an eyewitness, but the scene will definitely contain physical evidence that can provide help to the investigating officers (Lee and Pagliaro).

Classify and Characterize Evidence

• There are various ways to classify and characterize evidence:

  • Class characteristics: features that link an item to a group.

  • Individual characteristics: unique and associated only with its source.

• All evidence has class characteristics, but individual characteristics may require further analysis to identify or may not be identifiable on that specimen.

• Hair example:

  • Hair evidence can be considered class evidence if no root is attached.

  • If a root is attached and DNA can be obtained, it is individual evidence.

• Remember:

  • Class characteristics link specific items to a group.

  • Individual characteristics are unique and associated only with its source.

• Both class and individual evidence have value, but it typically takes more class evidence than individual evidence to link a suspect to a crime.