Physical evidence

Evidence: that which tends to prove or disprove something.

Q: Who decides the value of evidence in a criminal trial? A: A jury of 12 ordinary people.

So far we learned about testimonial evidence, which is oral or written statements given by people who witnessed an event. It can be useful in making a case, but it is unreliable. Fortunately, there is another kind of evidence that is much more reliable.

Physical evidence: Any evidence found at a crime scene in the form of a physical object.

Functions of a forensic scientist: analyze physical evidence. provide expert testimony. furnish training on the proper collection of physical evidence.

Uses of physical evidence: to link a suspect to a crime scene To link a victim to a crime scene To prove a crime has been committed

A forensic scientist is not on the side of the prosecution or the defense. A forensic scientist is on the side of the truth.

Physical evidence includes, but is not limited to, Fingerprints Footprints Documents Tool marks Bullets Computers Blood photographs Hair Fibers Soil Glass DNA

Classifying physical evidence by characteristics Class Characteristics Evidence associated only with a group is said to have class characteristics. Individual Characteristics Evidence that can be associated to a common source with an extremely high degree of probability is said to possess individual characteristics.

Class Characteristics … are physical qualities shared by a group of ‘like’ items, such as: the tread patterns on a brand of new athletic shoes the ABO typing of a blood sample (i.e.: AB negative)

Individual Characteristics … are physical qualities that are unique to an individual evidence item, such as: a pattern of cuts and wear marks on the tread patterns on a used pair of athletic shoes The DNA profile for a blood sample

Class characteristics are far more common than individual characteristics.

Class or Individual characteristics ? For each of the following pieces of evidence, say whether it has class characteristics or individual characteristics and explain why.

Class or Individual characteristics ? 1. The September, 2013, issue of Sports Illustrated magazine.

Class or Individual characteristics ? 2. A piece of a broken coffee cup.

Class or Individual characteristics ? 3. A hand-written grocery list.

Class or Individual characteristics ? 5. A new, unfired ammunition cartridge

Class or Individual characteristics ? 6. A fired bullet.

Class or Individual characteristics ? 7. A fingerprint.

Class or Individual characteristics ? 8. A wool fiber.

Other classifications of evidence Trace evidence: Physical evidence that is found at a crime scene in small but measurable amounts.

Other classifications of evidence Direct evidence: Evidence that (if true) proves an alleged fact, such as an eyewitness account or a confession.

Other classifications of evidence Circumstantial evidence: Evidence that implies a fact but cannot prove it directly. All physical evidence is circumstantial evidence.

As the number of different objects linking an individual to a crime increases, the probability of involvement increases dramatically.