 Today you will learn about the major categories of forensic evidence and how to determine which kinds of evidence fall into these different categories. There are four main classification schemes for evidence, physical versus non-physical, real versus demonstrative, known versus unknown, and class versus individual. So let's start with physical versus non-physical. Physical evidence includes any kind of tangible item from a forensic crime or accident scene. This category is what you normally think of when you talk about evidence, things like bullets, shoe prints, fingerprints, blood samples, fiber evidence, etc. Non-physical evidence includes items of evidence that are non-tangible, things like eyewitness testimony or an opinion from an expert witness. The next category of evidence is real versus demonstrative. Real evidence is a broad category that includes any evidence generated by criminal activity For example, blood spatter left by the criminal or shell casings left from a fired handgun. In other words, evidence created by the criminal at the scene of the crime. Some of this overlaps with physical evidence. Demonstrative evidence has any kind of representation of an object designed to present in the courtroom. In other words, things created for courtroom purposes after the event of a crime. Examples of this type of evidence would be photos, diagrams, x-rays, models, things like that. Another category of evidence is known versus unknown. Known evidence is evidence in which the source can be determined with certainty at the scene of the crime without later laboratory testing. For example, a blood sample taken from a victim at the scene of the crime. Individual evidence is when you do not know the source of the evidence without performing laboratory testing at a later date. Examples of this would be blood spatter evidence at the crime scene. You only know whose blood it is after you do DNA testing back at the lab. Last but not least, a very important category of evidence is individual versus class evidence. Individual evidence is evidence that can be linked with only one source. This source can be a person or even an object. Examples of individual evidence would be DNA samples, fingerprints, and specific wear patterns, including wear patterns on fired bullets, shell casings, tool marks, and shoe prints. Class evidence is evidence that is common to a group of objects or people and cannot be linked to a single source. Hair evidence is a great example of class evidence because you can never link it to a single source. Other examples include blood type evidence, hair evidence where you don't have the root which contains DNA, or shoe prints when you don't have individualizing wear patterns. So as you can see, some pieces of evidence can fall into multiple evidence categories. But it is important to understand the major distinctions between these classifications in forensic cases, especially the difference between individual and class evidence. It is critical that forensic scientists and jury members understand this distinction, as the ability to pinpoint evidence to a single person can make the difference between a conviction and an acquittal.