Evaluation and interpretation of crime forensic evidence

1. Evaluation and interpretation of crime forensic evidence Crime Trace recovery Potential • sources of the traces • scenarios producing the traces Evaluation And Interpretation

2. 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

3. 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.

4. 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

5. The parts of the process in mathematical form 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)

6. Using Bayes’ theorem on odds-form 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

7. 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)

8. What is the likelihood of Hpwhen 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

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

10. 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 window Hd: The glass fragments originate from another glass object (such as another window)

11. 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?

12. 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 window Activity level Hp: The suspect is the offender Hd: The suspect is not the offender Offense level

13. 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 ?

14. Example 1: Glass Activity: Smashing the window Fragments Being there before Transfer of glass fragments Garment under inspection Fragments recovered Fragments falling off

15. Network components: H “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 E “Match between recovered glass fragments and glass from smashed window”

16. T: Transfer node Should account for transfer, persistence and recovery of transferred fragments H T B B: Background node Should account for persistent fragments being there before activity and the recovery of these. E

17. Probability tables T Simplest case: Either were (one group of) n fragments transferred, persisted and recovered, or were 0 fragments transferred, persisted and recovered. B Simplest case: Either there were 0 fragments there before or were there (one group of) n persisting fragments there before.