A case from start to finish (direkt taget från Currans föreläsning)

1. A case from start to finish(direkt taget från Currans föreläsning) • In March 2004 a liquor store in Dunedin, New Zealand was broken into and approximately $300 worth of liquor was stolen. • To gain entry to the premises, the offenders smashed a window pane in a side door. • The two offenders were caught on video tape, but their facial features wereobscured by a “beanie” and a “hoodie”

2. Case details • The defendant, Mr Bill, was apprehended some hours later. • Mr. Bill’s outer clothing including a beanie was taken and searched. • Twenty fragments of glass were recovered from Mr. Bill’s beanie. • Mr. Bill had some explanations for the glass

3. The evidence • In line with SOP the lab determined the RI 10 of the 20 recovered fragments. • The lab also determined the RI of 10 control fragments from the scene • 7 of the 10 fragments “matched” the crime scene. • The 3 fragments that remained separated into two groups

4. Step one: Formulate the hypotheses • Contact: (HP ) Mr. Bill was in contact with the scene • Therefore, some of the glass found on Mr. Bill could have been transferred from the scene window to him. He also had two groups of glass on him prior to the breaking. • Contact: (HD) Mr. Bill was not in contact with the scene • Therefore, all of the glass found on Mr. Bill was from other sources and the “match” to the crime scene source was just coincidental

5. The denominator: Step 2: Derive the LR – denominator first The probability of the evidence given that Mr Bill was NOT at the crime scene • Possible reasons: • 3 groups of glass there before (of sizes 1, 7, and 2) • and one just happened to match the crime scene sample

6. The numerator: Step 2: Derive the LR – numerator The probability of the evidence given the Mr. Bill WAS at the crime scene • Possible reasons: • - No glass transferred, three groups of glass were there before and one matched the crime scene sample by chance or • - one group of glass transferred from the crime scene three groups of glass were there before and one matched the crime scene sample by chance

7. Step 2: Derive the LR

8. Step 3: Numerically evaluate the LR • Let’s look at lrcont first (Separat programvara, testas på datorövning) bill.xls

9. T - terms • From the video footage it was determined that the breaker was about 70cm from the window (d = 0.7) • A large number of fragments were transferred (=46) • One hour elapsed between collection of glass at the scene and confiscation of the beanie. Assume 2-4 hours may have elapsed (2≤t ≤4).

10. Assessing T • 70% to 80% of regular fragments were lost on average in the 1st hour, 70% thereafter (0.7≤p0≤0.8, pi=0.7 ) • 9% of fragments were high persistence (q=0.09) • 5% to 15% of hp fragments were lost on average in all hours since breaking (0.05≤p0,pi≤0.15) • 20% to 40% of fragments were recovered on searching • (0.2≤R≤0.4) • tfer.exe (separat programvara, testas på datorövning)

11. Step 3: Evaluate the LR numerically • T7 = 0.0171, so T7 lrcont=0.190 • From the sheet we get P2 = 0.0449, P3 = 0.0214 and S7 = 0.00587 • So