Aim: What types of evidence have probative value? The outcome of many criminal law cases will depend upon the strength and admissibility of evidence -- including physical proof, scientific evidence, and witness testimony. This lesson is part of a unit examining the probative value of different types of evidence.
Objectives: Students will be able to:
1) Explain Locard's Principle of Exchange.
2) Distinguish between physical evidence and trace evidence.
3) Understand why some types of trace evidence have more probative value than others.
Motivation: An eyewitness makes a statement about events that took place during a crime. Police collect physical evidence at the scene. For one reason or another, the two types of evidence don't add up. Which evidence is more reliable? Isn't trace evidence also subject to interpretation?
There are two types of evidence -- direct and indirect.
Direct evidence is evidence that proves a fact or proposition directly, rather that by secondary deduction or inference. Eyewitness testimony and a defendant's confession are both "direct" evidence.
Indirect, or circumstantial, evidence is a fact that can be used to infer another fact. Circumstantial evidence includes body fluids, fibers, and expert witnesses. For example, a woman is found dead at her home with a fiber on her shirt. Eventually, the fiber can lead to the killer.
Which of these is most useful in court, i.e, which has the most probative value?
Review the term probative value as it applies to different types of criminal evidence using the PowerPoint attached. Then, screen "The Value of Evidence" and have students complete the United Streaming worksheet as they watch the video.
"The concept known as the 'Locard's Exchange Principle' states that every time someone enters an environment, something is added to and removed from it. The principle is sometimes stated as “every contact leaves a trace”, and applies to contact between individuals as well as between individuals and a physical environment. Law enforcement investigators are therefore taught to always assume that physical evidence is left behind at every scene. This will be generally true, and the amount and nature of the evidence created will be largely dependent on the circumstances of the crime.
Examples include: Biological material - blood, semen or saliva; Fibers; Paint chips; Glass; Soil and vegetation; Accelerants; Fingerprints; Hair; Impression evidence – shoe prints, tire tracks or tool marks; Fracture patterns – glass fragments or adhesive tape pieces; and Narcotics.
"Oftentimes, evidence tells a story and helps an investigator recreate the crime scene and establish the sequence of events. Physical evidence can corroborate statements from the victim(s), witness(es) and/or suspect(s). If analyzed and interpreted properly, physical evidence is more reliable than testimonial evidence; testimonial evidence is more subjective in nature. An individual’s perception of events and memory of what happened can be incomplete or inaccurate. Physical evidence is objective and when documented, collected and preserved properly may be the only way to reliably place or link someone with a crime scene. Physical evidence is therefore often referred to as the 'silent witness.' "