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Evaluation and interpretation of crime forensic evidence. Crime. Trace recovery. Potential sources of the traces scenarios producing the traces. Evaluation And Interpretation. Roles. The commissioner (Police, Prosecutor, …) identifies potential sources and scenarios Hypotheses
Two simple hypotheses: Hpand Hd
Hp , “Prosecutor’s hypothesis” comes from the question: E.g. “Did this source leave the stain?”, “Was it this activity that gave rise to the recovered traces” …
Hd , “Defence’s hypothesis” . Is not necessarily formulated by the defence, but constitutes a non-overlapping alternative to Hp It can serve as the defence’s position.
Forensic findings: E (“E” for Evidence)
Prior odds: Quantifies the commissioner’s prior opinion
Posterior odds: Quantifies the commissioner’s opinion upon taking the evidence value into consideration
Value of evidence:
A likelihood ratio telling how much more (or less) probable the findings are given Hp than given Hd
Hp: “Mr X is the donor of the stain”
Hd: “Someone unrelated to Mr X is the donor of the stain”
“unrelated to” is needed here to make Hd simple (will be discussed later on in the course)
E : A match in DNA between the stain and Mr X (specific points about DNA analysis will be discussed later in the course)
The probability of getting a match if Mr X actually is the donor must be very close to 1 (deviations from 1 can only be explained by inaccuracies in the laboratory)
What is the likelihood of Hd when a match is obtained?
How rare is such a match? In a population of potential donors of the stain what is the proportion of persons sharing the current DNA profile?
When the DNA profiles compared are so-called complete , i.e. all markers investigated render approved analysis outcomes a very low proportion is expected, say 1 out of 109
interpreted as the match is 109 times more probable if Mr X is the donor of the stain than if someone unrelated to Mr X is the donor.
Can we then be certain that Mr X is the donor?
Assume a window has been broken in connection with a burglary.
One suspect, Mrs G has been identified. Clothes of Mrs G are confiscated for investigation.
On the trousers are recovered five fragments of glass.
Hp: The glass fragments originate from the broken window
Hd: The glass fragments originate from another glass object (such as another window)
Hp: The glass fragments recovered from the suspect’s jacket come from the smashed window
Hd: The glass fragments recovered from the suspect’s jacket come from another window
Hp: The suspect smashed the window
Hd: The suspect was never near the window
Hp: The suspect is the offender
Hd: The suspect is not the offender
Transfer of glass
“Hp : The suspect smashed the window”
“Match between recovered glass fragments and glass from smashed window”
Should account for transfer, persistence and recovery of transferred fragments
B: Background node
Should account for persistent fragments being there before activity and the recovery of these.
Simplest case: Either were (one group of) n fragments transferred, persisted and recovered, or were 0 fragments transferred, persisted and recovered.
Simplest case: Either there were 0 fragments there before or were there (one group of) n persisting fragments there before.