Evaluation and interpretation of crime forensic evidence

Crime Tracerecovery

Potential• sources of the traces• scenarios producing

the traces

EvaluationAndInterpretation

Roles

• The commissioner (Police, Prosecutor, …) identifies potential sources and scenarios Hypotheses

• Hypotheses should automatically be assigned prior probabilities

• A forensic laboratory analyses the traces (evidentiary material) and interprets and evaluates their findings with respect to the hypotheses Evidence values (measures of evidentiary strength)

• The commissioner combines the evidence values reported with the prior probabilities to obtain posterior probabilities for the hypotheses involved

Examples

Biological traces

• A saliva stain is recovered at a crime scene. The Police suspects Mr X to be the donor of the stain (i.e. that Mr X has spit at the crime scene)

• The hypotheses are “Hp : Mr X is the donor of the stain” and “Hd: Someone else than Mr X is the donor of the stain”

• The forensic laboratory analyses the stain with respect to its contents of DNA and compares with DNA obtained from swabbing of Mr X

• The findings may be that there is a match in DNA• The match is interpreted and evaluated by investigating the rarity of

the DNA profile in a proper population of potential donors of the stain.

• If the evidence value is high enough the Police may conclude that Mr X is the donor. The final decision, however, is made at court.

Cross-transfer

• A blue car was hit by another car that escaped the accident• Traces of red car paint was found at the point of collision• Later on a red car was found and investigated. That car had a mark

potentially from a hit where fragments of blue car paint were recovered.

• The Police suspects that this red car was the car that hit the blue car.• The forensic laboratory analyses the paints (red and blue)

recovered. They find that the red paint recovered on the blue car matches the paint of the red car and that the blue paint recovered on the red car matches that of the hit blue car.

• The damage marks on the cars are investigated to see whether the transfer of car paint in both directions is probable or not.

• The rarities of the two types of paint are investigated.• A value of evidence is estimated telling to what extent the findings

support that the red car actually hit the blue car• The Police combines the evidence value with their prior opinion

about whether the red car hit the blue car

The parts of the process in mathematical form

Two simple hypotheses: Hp and 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)

Using Bayes’ theorem on odds-form

d

p

d

p

d

p

HPHP

EHLEHL

EHPEHP

Posterior odds: Quantifies the commissioner’s opinion upon taking the evidence value into consideration

Prior odds: Quantifies the commissioner’s prior opinion

Value of evidence: A likelihood ratio telling how much more (or less) probable the findings are given Hp

than given Hd

Examples

Saliva stain:

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)

What is the likelihood of Hp when a match is obtained?

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

1 EHL p

99 10101 EHL d

Hence, the likelihood ratio, i.e. the value of evidence is

99 10

101 EHL

EHL

d

p

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?

Glass fragments

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.

Hypotheses:

Hp: The glass fragments originate from the broken windowHd: The glass fragments originate from another glass object (such as another window)

How can these items of evidence be analysed by a forensic laboratory?

• Visual inspection of fragments with respect to colour and thickness• Measurements of refractive index on fragments and on glass from

the broken window.• Measurements of the elemental composition (made by scanning

electron microscopy techniques)

What is E?

The hierarchy of propositions

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

Source level

Hp: The suspect smashed the window

Hd: The suspect was never near the windowActivity level

Hp: The suspect is the offender

Hd: The suspect is not the offenderOffense level

The activity level

• Depicts an activity from which the evidence has emerged• Is not necessarily a criminal activity Should most often be

considered by the forensic scientist• Evaluation of evidence under activity propositions needs information

about circumstances with the crime

?

Example 1: Glass

Activity: Smashing the

windowTransfer of glass

fragmentsGarment under

inspectionFragm

entsfalling off

Fragments

Being therebefore

Fragmentsrecovered

Network components:

H

E“Match between recovered glass fragments and glass from smashed window”

“Hp : The suspect smashed the window”

Nodes to be added taking into account:• transfer of fragments from window to garment• persistence of fragments on garment• persistent fragments on the garment prior to activity

H

T

E

B

T: Transfer node

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.

Probability tables

TH Hp Hd

T Tn tn 0

T0 1 – tn 1

BB Probability

B0 b0

B1,n b1,n

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.