8127/frame/fm

 
c.
When Is Evidence Class Evidence?
Because a class can contain as few as two members, just about any type of
evidence may be encountered as class evidence. Even potentially individual-
izing evidence must still be treated as class evidence if individualizing traits
are not available for comparison in both the reference and evidence samples.
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Principles and Practice of Criminalistics
All types of trace evidence must frequently be treated as class evidence. Fibers,
even if rare or unusual, are manufactured in quantity, and therefore are not
individualizable strictly by composition. Hairs, as examined microscopically,
make for some of the weakest class evidence. In fact, many analysts believe
that hairs should only be used as exclusionary evidence. In the absence of a
physical match or some extremely unusual characteristic, glass can only be
assigned to a relatively large class. Paints and polymers are class evidence by
composition; however, the presence of layers introduces the possibility of
individualization. The gross rifling characteristics of firearms and fresh-
from-the-factory tools only provide class evidence. Finally, serological blood
grouping only provides class characteristics, although the class can be nar-
rowed quite substantially using a combination of several markers.
 
C.
Identification
Because individualization is perceived as the ultimate goal of a forensic exam-
ination, the inability to individualize is sometimes equated with failure. This
is unfortunate, for it overlooks aspects of criminalistics that unquestionably
make up the largest volume of the forensic community’s work. Recall that
identification classifies materials into categories where more than one object
shares the same characteristics. These could be yellow pencils, .22 caliber
cartridges, size 9 shoes, or all heroin molecules. The reader can, no doubt,
think of many similar examples.
We have mentioned several times that identification frequently suffices
to meet a legal standard of proof. This is evident in crimes that involve illicit
substances, namely, drugs, where mere possession of specified chemicals is
deemed illegal; one need only demonstrate that a class of molecules known
as heroin is present. Another example is the assessment of DUI (driving under
the influence) where the finding of a particular concentration of alcohol in
a person’s blood, breath, or urine, by definition, legally prohibits the opera-
tion of a motor vehicle. The molecules of ethyl alcohol in the person’s system
are the same as any other molecules of ethyl alcohol; it is their presence above
a predetermined concentration while driving that is illegal. Interestingly,
these limited situations are the only time that physical evidence can be
considered 
 
direct evidence
as opposed to 
 
circumstantial evidence
. The mere
finding of cocaine is direct evidence of criminality; the fact-finder requires
no inference or assumption to reach this conclusion.