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Types of Evidence

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Evidence can be divided into two categories:

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:

Human hair with root still attached

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."

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NOTE TO USERS: The President’s DNA Initiative DNA Analyst Training program and assessment were completed and published in 2005, in cooperation with the National Institute of Justice. The science and techniques in the program are sound and proven, however, program content has not been updated to include tools and technologies developed and in use after 2005, including many kits and robots. Assessment questions address only content delivered in this program and may not contain the full range of tools in use in your laboratory.