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D.
Evidence and Reference
A characteristic aspect of forensic science that differentiates it from all other
applications of science is the comparison of evidence samples to reference
samples. This criterion was recognized by at least one judge in attempting
to determine the qualification of a defense expert to comment on forensic
casework (
 
People v. Maclanahan
, 1998). The relationship between an evidence
item and a reference item is operational. Like everything else, it depends on
the question. A bloodstain on the sheet of a murder-rape victim functions
as an evidence item until we verify that the source is the victim. It may be
used as a secondary reference with which to compare blood on a suspect’s
pants 20 years later. Nevertheless, for most kinds of evidence (as used collo-
quially), the correlation between an evidence item and a reference item is
pretty obvious. It is hard to mistake the spent cartridges at the scene of a
shooting as anything other than evidence samples and the cartridges cycled
through the suspect weapon’s action are clearly reference samples. The paint
transferred to a car’s fender is clearly an evidence item and the car from
which it came a reference item, regardless of who was at fault.
Less evident, perhaps, is which item, evidence or reference, one should
use to define the traits on which to base a comparison. In large part, this
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Principles and Practice of Criminalistics
judgment impacts decisions you will make during analysis, and a correct
understanding is also crucial to the reasoned collection of evidence at the
scene. In particular, the concepts are helpful to understand the connection
of trace evidence both to the scene and to possible reference samples. Because
hair and fibers are ubiquitous in our environment, it is often difficult to
determine which, if any, relate to the crime and which are simply adventi-
tious. In desperation, the criminalist will sometimes survey the suspect and
his environment (house, car, employment) for reference material, and then
look at the crime scene samples for matching evidence. Stoney (1984) has
made the case that this is backward; defining the relevant hairs, particles, or
fibers by looking for reference material in crime scene evidence emphasizes
the reference rather than the evidence. By focusing on reference samples
gathered according to a particular suspect, we ignore the possibility that the
trace material may be adventitiously associated with the crime, or that some
other reference is the source; worse yet, we may miss detecting some truly
relevant trace evidence. The most relevant evidence is that which is patently
related to the crime event. Items adjudged to be evidence by first surveying
the environment of a suspect interject an element of bias into both the
investigation and the analysis. Criteria for concluding the evidentiary value
of trace material include abundance, rareness, and lack of representation in
the victim’s environment. This process is perhaps the ultimate challenge in
the recognition of evidence.