8127/frame/fm

6.
Who Makes the Inference?
A significant and reoccurring issue is, who makes the inference of contact,
the scientist or the fact finder? It is the purview of the fact finder to draw
inferences from circumstantial evidence, and, of course, potentially individ-
ualizing physical evidence is circumstantial evidence. However, there are
pieces of information that only science can legitimately provide to the fact
finder, such as population frequencies, transfer and persistence data, and
limitations of the evidence and the test. These clearly are of assistance to the
Posterior odds = Prior odds
Prior odds
P
the defendant murdered two people
(
)
P
the police planted the blood
(
)
---------------------------------------------------------------------------------------------
Small number
Big number
---------------------------------
=
=
Small number
Big number
---------------------------------
1
×
P
the defendant murdered two people
(
)
P
the police planted the blood
(
)
---------------------------------------------------------------------------------------------
Posterior odds
=
=
8127/frame/ch07 Page 176 Friday, July 21, 2000 11:46 AM

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Association and Reconstruction — Inference of Contact
177
fact finder in drawing inferences about physical evidence. We assert that the
scientist is in the best position to provide this information and to place it
within the context of the case. When communicated to the fact finder in a
manner that clearly establishes the limits of the information, the scientist can
and should talk about an inference of contact. The scientist must avoid
statements about guilt or innocence, and should clarify the assumptions upon
which the inference relies. As assumptions change, the inference may also
change, and the scientist must be willing to adjust her inferences as infor-
mation changes.