Scientific Method Applied to Forensic Science

Criminal Justice - Forensic Science


The Scientific Method and Comparative Bullet Lead Analysis:

Generally, the scientific methods consists of forming and testing hypotheses to explain an observed phenomenon or group of phenomena. More specifically, scientific inquiry consists of disprovable hypotheses capable of being tested in relation to corresponding phenomena or quantitative predictions in ways that can either establish a logical basis supporting the proposed explanation or refute it. Finally, scientific tests must be repeatable by other experimenters. In criminal investigations, the scientific method underlies every component of forensic analysis. Furthermore, aside from the requirement that any method used to establish evidence of a crime be supported by a valid, testable, repeatable hypothesis, the burden of proof on the prosecution also requires any relevant test used to establish guilt be sufficiently accurate to a very high degree of certainty (Kobalinsky and Liotti, et al., 2005).

One of the most dramatic examples of the use of the scientific method involved the testing of the validity of a sophisticated modern forensic techniques used to establish guilt in hundreds of criminal cases since the 1960s. The results of those tests resulted in the invalidation of comparative bullet lead analysis, a test used extensively by the Federal Bureau of Investigation (FBI) to establish the identity of criminal defendants accused of crimes involving firearms. Comparative bullet lead analysis was first used in 1963 in connection with the assassination of President John F. Kennedy (Solomon, 2007). In principle, the technique is predicated upon the assumption that bullets manufactured within the same batch bear statistically relevant similarity that distinguishes them from other bullets.

In theory, the chemical composition of various measurable differences within bullets, the technique could positively identify bullets recovered from crime scenes and link them definitively to other bullets recovered from suspects. This method is extremely useful to law enforcement investigations, particularly in situations where insufficient evidence is available on bullet fragments for gun-barrel groove marking analysis. The entire prospect of using comparative bullet lead analysis in criminal prosecution was successfully challenged on scientific grounds in 2002, and internal FBI documents disclosed during that investigation suggested that the bureau had expressed similar concerns for more than a decade (Solomon, 2007).

Hypothetical Framework and Comparative Bullet Lead Analysis Methodology: The hypothesis behind bullet lead analysis is quite simple: namely, that specific elements in the lead in bullet fragments vary consistently between different batches of lead used in their manufacture and that those differences are capable of being measured precisely enough to exclude bullets from having come from the same box of ammunition by virtue of the chemical analysis of their specific lead content. The hypothesis arose from the fact that American bullet manufacturers typically use large quantities of lead obtained from melting down recycled car batteries. The expectations was that the large batches of lead used by each manufacturer to produce the ammunition within boxes manufactured at approximately the same time would yield individual bullets of much closer chemical composition than boxes manufactured from different batches of lead (Solomon, 2007). The hypothesis is perfectly analogous to the methods used to establish paternity using DNA information.

Instead of genetic information extracted from biological tissues, comparative lead analysis relies on the measurable differences of various trace chemical elements, such as antimony, arsenic and copper (Solomon, 2007).

In its initial incarnation, this differential chemical analysis was conducted via the process of directing neutron beams at the bullet fragments. In 1996, the neutron beam method was replaced by a form of optical spectrum emission spectroscopy that the bureau calls "inductively coupled plasma optical emission spectroscopy." The principal benefit of the new method was that it provided measurements of four more trace elements in the bullets, adding bismuth, cadmium, tin, and silver to the original three elements.

One of the other expected advantages of using seven markers instead of three was increased precision (Solomon, 2007).

While the FBI does not have (federal) jurisdiction over criminal investigations of individual states, the bureau nevertheless assists local criminal investigations with data and forensic analysis.…