Advances in Fingerprint Technology.pdf

is 1/2 that the identification is valid; and if three individuals, the chance is
1/3; etc. Kingston’s calculation answers the following question:
Given a fingerprint from an unknown source, and assuming an individual
is selected randomly from among all persons who actually have the fingerprint
pattern on their fingers, what is the probability that this one individual
made the fingerprint?
The situation is analogous to having all N persons who can make the
print present in a closed room. One of the persons inside is randomly
selected, and we ask for the probability that this person is the actual source
of the print. We can but note that the person is one of the possible sources
and that the probability is 1/N that we have the correct person. The frequency
of the fingerprint determines the magnitude of N, the number of persons in
the room.
Contrast this situation with one in which the individuals within the room
are selected randomly with respect to fingerprint type, and where we test the
individuals to determine if they could have actually made the evidence print.
The number of persons in the room (N) now represents a population of
suspects to be tested using the evidence print. If fingerprints of an individual
in this suspect group match the evidence print, what is the significance of
this finding? This question parallels the practice of fingerprint comparison,
whereas Kingston’s does not. Kingston had assumed that his suspect has been
selected solely on the basis of correspondence with the fingerprint. Rarely
would this be the case; most often, identification by a partial fingerprint
would be a nearly independent event. The comparison would be used to test
a few possible suspects rather than to define the suspect group. When many
suspects are screened by means of the fingerprint, the chances of false association
rise, as pointed out by Amy. Only in the hypothetically absurd
extreme, in which the entire population of the world is screened, is Kingston’s
calculation valid.
To answer the appropriate question posed above, one must compare the
chance of the evidence occurring under two hypotheses:
H 1: that the individual in fact made the print
H 2: that another (random) individual made the print
Under H 1, it is certain that the print would match the individual. Under H 2,
the probability is the frequency of incidence multiplied by the number of
attempts made to compare the print. A likelihood ratio of these two probabilities
gives the relative support of the evidence for the two competing
hypotheses. 45