How are forensic tests performed in a laboratory?

Before carrying out any laboratory work, it’s crucial to ensure the workspace is clean and free of contamination. Then, the evidence is visually inspected and properly described to document its condition. Often, they are photographed, weighed, and sketched. Following this, the laboratory worker (criminalist, forensic lab technician, or forensic scientist) must determine which tests are appropriate, whether there are sufficient quantities of evidence, dissect the part to be analyzed correctly, and properly prepare the analysis material (which may include the delicate mixing of numerous chemical compounds), all while documenting every step.

Only then does the testing begin. Some tests involve up to five or six different procedures, each of which must be performed and documented correctly. The evidence must be repackaged and labeled correctly, and once again, transported to storage. Only then does the laboratory worker start the process of interpreting what the experiments have revealed.

A report is drafted, the content of which must be exactly correct. At the prosecutor’s discretion, the evidence has to return to the police evidence room, where it will be stored until they decide they want to use it or that more tests should be conducted, in which case it returns to the forensic lab. It should be quite evident that all this transportation of evidence leads to numerous possibilities for error in the form of destruction, mishandling, and contamination.

Quality assurance refers to the measures a lab takes to control, verify, and document its performance. A basic business principle is that quality assurance serves to check on quality control. Protocols consist of several hundred pages of very technical manuals and must include aspects such as “validation studies” that the lab itself conducts to be capable of performing tests in a specific discipline. Proficiency tests determine whether individual laboratory workers, and the laboratories as institutions, meet professional standards.

In these tests, samples to be examined are given to a laboratory or a specific worker, but the results are already known by the tester. There are two methods employed in administering these tests, blind and open. In the blind test, the laboratory worker does not know that a test is being conducted; they think the sample they are working with is just another case. The open proficiency test is like an open-book exam. Additionally, laboratory workers join associations to enhance their resumes.

Physical evidence is also the main provider of extraordinary clarifications, in which the police can link different crimes at different times and places to the same offender. Working with physical evidence means being aware at all times of what the prosecutor needs to win the case in court.

Undoubtedly, the most important concept in forensic science is identification, or what Paul Kirk called individualization (Kirk 1936). Identification produces an unambiguous (certain) interpretation. An ambiguous crime scene, in comparison, produces physical evidence subject to different interpretations (and is the basis for crime reconstruction and profiling).

When a series of details come together (as in comparison points), such that they constitute a class of one (by itself), it is said to establish an identity, also called individual characteristics, or entities of a class by itself. At this point, if there are similarities between the crime scene evidence and evidence from a suspect, the expert can state, without a doubt, that the identity has been individualized.

It cannot be said that the investigator has “individualized” anything at this point, but the red hair in this example may be considered a sufficient individual characteristic that combines with the class characteristic to establish identity. At this point, if there’s a “match” between fibers and hair from the crime scene with a suspect, the examiner can say that the crime scene sample (unknown or questioned) “may come from the same source” as the suspect’s sample (known or standard), which is enough for probable cause.

Identity can also be used to conclusively eliminate individuals as suspects. Class characteristics alone do not allow matching with a single suspect. Matches of evidence with individual characteristics do allow pointing to a specific suspect. Matches at the individual level are called similarities because, in theory, a perfect match does not exist. An example of a similarity, or match in individual characteristics, would be a number of comparison points between the friction ridge lines of a latent fingerprint (from the crime scene) with the fingerprints of a specific suspect.

This type of true match allows the examiner to state that, since the similarities outweigh any differences, the sample from the crime scene (unknown or questioned) “comes from the same source” as the suspect’s sample (known or standard), which is sufficient, most of the time, for proof beyond a reasonable doubt. Reliability and integrity increase if the examiner has performed tests on patterns using controls. A standard is the opposite of a standard.

A standard always comes from a suspect or something that represents a known modus operandi or signature. Standards consist of simultaneous tests conducted with items taken from the “background” or vicinity of the crime scene (control sample), items similar to those used by the suspect, such as test shots from a similar weapon (standard sample), or items used for calibration purposes kept in stock in the lab (reference sample). The astute student will realize that the word “match” is used in two different ways in forensic science. Some prefer the term “matching” (Ramsland 2001). The misuse of the term “match” is even worse in forensic science, allowing different disciplines to set their own criteria and standards.