Standards of Forensic Science

Pricilla Hernandez Aguilar

Course Standard 3.1

Locard's Exchange Principle



  • Locard’s Exchange Principle: Holds that the perpetrator of a crime will bring something into the crime scene and leave with something from it, and that both can be used as forensic evidence.
  • developed by Dr. Edmond Locard (1877-1966)
  • Locard hypothesized that every time someone made contact with another person, place, or thing, it results in an exchange of physical materials.
  • He also believed that no matter where a criminal goes or what a criminal does, by coming into contact with things, a criminal can leave all sorts of evidence, including DNA, fingerprints, footprints, hair, skin cells, blood, bodily fluids, pieces of clothing, fibers and more.
  • His exchange principle has been a greatly influential piece of work in forensic science, and is frequently quoted to this day.
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Frye Standard

  • Frye Standard/ Frye Test: Or General Acceptance test is a test to determine the admissibility of science of scientific evidence. It provides that expert opinion based on a scientific technique is admissible only where the technique is generally accepted as reliable in the relevant scientific community.
  • basically states that scientific evidence presented to the court must be generally accepted, by an association with the scientific community as interpreted by the court
  • Originated from the case Frye v. United States which was a murder case in 1923.
  • It was a case which involved the criminal defendant that was convicted based on a lie detector.
  • The defendant ended up arguing that the technique used was not recognized by scientist in the field, and the court agreed .

Daubert's Ruling:

  • Daubert Ruling: Standard provides a rule of evidence regarding the admissibility of expert witnesses’ testimony during United States federal legal proceedings.
  • Standard used by a trial judge to make a preliminary assessment of whether an expert’s scientific testimony is based on reasoning or methodology that is scientifically valid and can properly be applied to the facts at issue
  • Under this standard, the factors that may be considered in determining whether the methodology is valid are....


  1. whether the theory or technique in question can be and has been tested
  2. whether it has been subjected to peer review and publication
  3. its known or potential error rate
  4. the existence and maintenance of standards controlling its operation
  5. whether it has attracted widespread acceptance within a relevant scientific community

  • Origninated from Daubert v. Merrell Dow Pharmaceuticals Inc, the U,S court changed the standard for admissibility of expert testimony

Course standard 3.2

Different Types Of Evidence

  • Testimonial Evidence : Oral or written assertion offered in a court as a proof of the truth of what is being stated. It includes testimony and hearsay evidence. ( Can also be called “Direct” or “Prima facie” Evidence.)
  • Physical Evidence : Usually involves objects found at the scene of a crime. Physical evidence may consist of all sorts of prints such as fingerprints, footprints, handprints, tidemarks, cut marks, tool marks, etc. (Can also be called “real” evidence.)
  • Physical evidence is usually more reliable than testimonial evidence. Why ? Because it can prove many things such as : establishing identity of a person(s) involved in the crime, Prove crime was committed, link a suspect with a crime scene/victim, and can also help reconstruct events of crime. The actual difference between testimonial and physical evidence is that….. Testimonial is basically only a statement given by the accused/witness/ expert under oath. Physical evidence is anything found at the crime scene that is an object or something materialistic.

  • Types of Physical evidence are transient, pattern, conditional, transfer, and associative evidence.


  1. Transient evidence is temporary; easily changed or lost usually observed by the first officer at the scene
  2. pattern evidence is produced by direct contact between a person and an object or between 2 object.
  3. Conditional evidence is produced by a specific event or action,important on reconstructing and determining the set of circumstances or sequence within a particular event
  4. Transfer evidence is produced by contact between person(s) and object(s) or between person(s).
  5. Associative evidence is something that may associate a victim or suspect with a scene or with each others.

Individual and Class Evidence :

  • Individual Evidence : A kind of evidence that identifies a particular person or thing
  • Individual evidence shows an object's uniqueness
  • Can specifically place things to a person or a place or an object to the exclusion of all others
  • Examples are DNA, fingerprints, some footwear impressions, bite marks, bullets
  • Class Evidence : Material that connects an individual or thing to a certain group
  • Class evidence can place an object to a group, but not a an individual
  • Includes a group of suspects
  • Examples are fibers, paint, soil, glass

Course Standard 4.1

Process Of Performing An Autopsy

  • Definition : A postmortem examination to discover the cause of death or the extent of disease
  • An autopsy is conducted by a pathologist
  • Autopsies are performed for a variety of reasons..


  1. determine cause of death
  2. check if clinical diagnosist are correct
  3. provide closer for family ect..


  • In the U.S., an autopsy can be ordered by a coroner or medical examiner if there are suspicious circumstances surrounding the death


  1. First, note the height, weight, age, and sex of the body. Note any distinguishing characteristics like birthmarks, scars, or tattoos as well.
  2. An X-ray will help you to find any broken or fractured bones, or medical devices, such as a pace-maker.
    These records can also be used to identify the subject
  3. Check the genitial area for any signs of rape. Bruising and tearing are common in such cases.
  4. Take a blood sample.It can be used for DNA purposes, or it can help to determine if the victim was on drugs, had been using alcohol, or whether there was poisoning involved.
  5. Open the body cavity once the initial examinations are complete. Using a scalp make a one large "Y" shape incision starting from each shoulder across the chest all the way down the pubic bone. Once that is done you spread the skin and check the insides and see if any ribs are broken
  6. Examine each organ in the chest cavity individually. The organs are weighed and recorded and tissue sampes are taken on futher examination if needed.
  7. Then observe the eyes carefully.
  8. Then you look at the head. Check for any Trauma to the skull, including fractures or bruises. Then remove the top of the skull, and remove the brain.
  9. State the cause of death, and the reasons that brought you to that conclusion.