8127/frame/fm

 
b.
Items with the Same Class Characteristics May Have 
Different Origins
Perhaps obvious, but worth exploring, are the implications of the one-evi-
dence-from-many-possible-sources conclusion inherent in class evidence.
(Figure 6.2). Because multiple source items exist, any one of them could be
a potential source of some matching evidence item. For instance, millions of
tires may be cast from a single mold, all having the same width and tread
design. Frequently, these are the only characteristics that we can discern from
a tire track. Based only on these class characteristics, any one of those millions
of tires (or at least the thousands in the geographic area at the time) could
be the source of the track.
We began this discussion with the class characteristics of shoes and their
prints. Consider the following hypothetical:
A partial shoeprint is found in dust. The print contains some detailed
characteristics from the shoe, including width (but not length), the logo of
the shoe, and some tread detail. By doing some research, we find out that
a shoe with these class characteristics has a polyurethane sole and was
8127/frame/ch06 Page 119 Friday, July 21, 2000 11:47 AM

---

120
Principles and Practice of Criminalistics
 
Figure 6.2
 
One to many; one to one.
When only class characteristics are present
in an evidence item, it is not possible to determine a single source for it. The
evidence item could also originate from many other objects with indistinguish-
able class characteristics. Therefore, one evidence item could have many possible
sources (classification). When an evidence item shows several individualizing
traits, the analyst may conclude that only one object could be the source of the
evidence. The analyst believes that only one object with indistinguishable indi-
vidualizing traits exists. In this circumstance, the evidence item has only one
possible source (individualization). It is interesting to note that we virtually never
compare an evidence item to its putative source, but rather to a known reference
from that source: a bullet from a gun, a print from a finger, a blood sample from
a person.
GENERIC
SHOE
GENERICSHOE
GENERIC
SHOE
GENERIC
SHOE
GENERIC
SHOE
GENERIC
SHOE
GENERIC
SHOE
GENERICSHOE
GENERIC
SHOE
GENERIC
SHOE
Classification
Individualization
one
to
one
one
to
many
REFERENCE
EVIDENCE
8127/frame/ch06 Page 120 Friday, July 21, 2000 11:47 AM

---

Classification, Identification, and Individualization
121
manufactured by the Shoes-’R-Us Company for 16 months, after which the
molds were destroyed. They tell us that approximately 10,000 of these shoes
were made in sizes corresponding to the measured width.
A possible suspect is developed, and investigation uncovers a pair of
size 14 shoes of this make and style in his closet. Comparison to the reference
print made with the suspect’s right shoe shows concordance with all of the
class characteristics visible in the evidence print. Assuming no individual-
izing traits are evident, what can we say about the relationship of the
suspect’s shoe and the evidence print?
In this not-uncommon situation, it is incumbent upon the analyst to
clarify both the hypothesis and the alternative hypothesis, and to consider
the evidence in light of both. The hypothesis is that the suspect’s shoe is
the source of the print; an alternative hypothesis is that some other shoe
bearing these class characteristics is the source of the print. Any conclusion
about the relationship of the shoe and the print must communicate not
only the information about the match, but its limitations, and at least an
estimate of its strength. Without all of these elements, the conclusion is at
best incomplete, at worst misleading.
Sometimes, we need to consider a hierarchy of class characteristics. For
instance, clay mined for pottery will be distributed from the quarry to many
different pottery factories. Consequently, the pottery pieces from all of the
factories will exhibit similar class characteristics when analyzed for mineral
composition. Mass-produced pieces from each different manufacturer may
also acquire an additional set of class characteristics particular to molds or
paints used at a certain location. Firearms examiners have identified these
kinds of traits in certain types of barrel manufacture, and have termed them
 
subclass characteristics
(Nichols, 1997). Thornton has also discussed this issue
and its value to the forensic examiner (Thornton, 1986b).
Determination of source depends both on the question and the state of
the evidence. Suppose we have an entire pot with a very intricate relief design
characteristic of a single pottery manufacturer. With this information, we
could answer questions about where the pot was made and from which
quarry the clay originated. In contrast, suppose we have only a small fragment
of broken pottery. No other pieces of the pot are available. Although we might
trace the clay back to the quarry, we would be unable to place the fragment
in the smaller class of pots from one particular manufacturer.