With the exception of nuclear DNA analysis, however, no forensic method has been rigorously shown to have the capacity to consistently, and with a high degree of certainty, demonstrate a connection between evidence and a specific individual or source.
Many disciplines that make up the forensic sciences have the scientific method at their core. The research for the National Academy of Sciences report, A Path Forward examined how the scientific method was applied, and described the disciplines by subjective and objective measures. The report stated, “In terms of scientific basis, the analytically based disciplines generally hold a notable edge over disciplines based on expert interpretation”. Yet it also pointed out that due to the variability in the application of technique and interpretation of data, exact determinations could not be made regarding reliability and validity, either within or across disciplines.
Many forensic disciplines depend upon technology or at least use it in some way. In most cases, the technologies employed provide a method of determining or measuring something at a high level of exactness.
Many forensic techniques also draw from what could be considered traditional lab sciences, such as chemistry or biology. For instance, DNA profiling relies on technology, but its premise is based on molecular biology and molecular genetics. Forensic engineering is largely technology-based, and relies on sound principles of physics. And drug identification chemistry is based on principles of organic and analytical chemistry. Many disciplines of forensic science are deeply rooted in the scientific method.
Forensic science disciplines fall loosely into two categories and span a spectrum of scientific objectivity.
Some forensic science disciplines are more art than science, requiring a fair amount of human interpretation to generate results. But practitioners all along the spectrum are expected to use the same level of care, adhering to the hypothesis testing method, and not deviating from accepted protocols and examination procedures.
Every discipline puts forth large bodies of published research, and delineates vast amounts of experience of large numbers of practitioners, supporting a claim that the discipline is a legitimate science. The difficulty for the courts is how to make sense of all this data. Does the empirical data and reasoning support the specific claim being made by the expert?
Consider the complexity of the subjective-objective spectrum by delving into some select disciplines in the following sections.