8127/frame/fm

ii.
Inference of contact.
The strength of an inference of contact concerns
the relationship of different items of evidence to each other. It incorporates
information about the evidence in the context of the case. With a consider-
ation of contact, we consider the relationship of 
source to target
(association).
An inference of contact starts with the forensic scientist in the laboratory and
extends to involve eventually others connected with the case, including the
investigators and attorneys. Note that, for an association, the strength can only
be as good as the strength of the inference of source upon which it is built.
d.
A Statement of Conclusion
Finally, the analyst must provide a conclusion that will communicate the
meaning of the physical evidence results to any interested party who reads
the report. A conclusion is an inference (of either source or contact) accom-
panied by a statement of assumptions and limitations. For identification
evidence, the assumptions and limitations are usually trivial and the conclu-
sion is simply a statement regarding the nature and quantity of the substance
detected. For individualized evidence, a statement of conclusion about source
The final interpretation might be summarized as follows:
1.
More than one individual contributed to the blood samples collected from the console of the
Bronco.
2.
All individuals carrying the 2 and 3 DQ
α
alleles are eliminated as contributors to the detected
DNA. All others are included as possible donors.
3.
Individuals included in the evaluation in step 2 are eliminated if they do not have some
combination of the D1S80 18, 24, and/or 25 alleles. The remainder are included as possible
donors.
The genotypes of the three reference samples each contain some combination of the DQ
α
1.1, 1.2, 1.3,
and/or 4 alleles. Each also contains at least one of D1S80 18, 24, and/or 25 alleles. Thus, none of the
three principals is eliminated as a possible contributor to this sample.
Another way of evaluating the results would be to examine combinations of types from the reference
samples to see if any combination could produce the pattern seen in the evidence stain. The Bronco
console stain(s) cannot be just a mix of NB and OS, they both lack the DQ
α
1.3 and 4 alleles. All other
combinations are possible (NB/RG; OS/RG; NB/OS/RG). Similarly, the stain(s) cannot be a mix of only
OS and RG; neither carries the D1S80 18 allele. The stain(s) also cannot be a mix of only NB and RG;
neither has the D1S80 25 allele. Other combinations cannot be eliminated (NB/OS; NB/OS/RG).
Thus all pairwise combinations of the reference genotypes are eliminated by the results from either
one or the other marker system. Only a mixture of all three could account for the evidentiary pattern.
Therefore, the stain was either contributed by a mixture of all three principals, or by two or more unknown
individuals. Likelihood ratios would assist in determining which of these hypotheses was a more likely
cause of the evidence.
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Good Forensic Practice — Obligations of the Analyst
269
determination is also straightforward; the analyst simply states her opinion
that the evidence and reference could only originate from a unique common
source. The assumptions and limitations are usually summarized in the words
“in my opinion.” It is for classified evidence where the formulation and
wording of a conclusion make a real difference in how a reader will under-
stand the import of your results.
The field has not agreed on consistent standards or wording for conclu-
sions about classified evidence. The following conclusions are commonly
encountered in reports about classified evidence:
• The reference cannot be excluded as the source of the evidence.
• The reference is included as a possible source for the evidence.
• The reference is a possible source of the evidence.
• The evidence could have come from the reference.
• The evidence is consistent with coming from the reference.
• The evidence and reference have matching class characteristics; there-
fore, they could have a common origin.
• The evidence and reference are consistent with originating from a
common source.
Although none of these statements contains any blatantly false conclusions,
the strength of the conclusion is conveyed slightly differently in each. First,
none of these statements addresses the possibility of the evidence originating
from a source other than the reference. In addition, they all lack any quan-
titative or semiquantitative information that would assist the reader to under-
stand the strength of the source determination. The omission of any
statement of strength renders all of the above statements incomplete. A
statement that incorporates an estimate of the number of possible sources
of the evidence would provide a much better understanding to readers of the
import of the findings.
Also lacking from any of the above statements is an articulation of any
alternative hypotheses. This is incomplete at best, misleading at worst. A
likelihood ratio provides a vehicle to compare the simplest alternative
hypothesis, that the evidence derives from another source with similar char-
acteristics, and also to consider more complex scenarios.
We leave you with our favorite comment on classified evidence by Jim
Trotter III, the district attorney played by Lane Smith in the movie 
My Cousin
Vinnie
. A portion of the text can be found in Figure 10.3.