4.4 Case scenario: forming propositions

Police were called to the house of a known local drug dealer after reports by neighbours of a suspected gunshot heard from within the property. They arrived at the scene to find a middle-aged male, the known drug dealer, dead in a bedroom with gunshot wounds. A window at the back of the house on the ground floor was found to be smashed, with the glass having fallen towards the inside of the house. As part of the investigation, they found death-threats on the deceased’s public social media account from two males. The males are known to police, having previously been involved in drug-related crimes. Both males are arrested at their known addresses, at which time Suspect 1 confesses to having broken in to the house but not having fired the weapon. He refuses to name any accomplices. Suspect 2 refuses to comment, despite glass-covered clothing being found within a plastic bag in a bin at his residence. After further testing, the clothing is found to be covered in gunshot-residue particles. No weapon is recovered.

Here are three facts-in-issue of the many for this case at this preliminary stage.

Fact-in-issue 1: whether or not Suspect 2 killed the victim with the intent to kill or cause grievous bodily harm

• \(H_p^1\): Suspect 2 committed murder,
• \(H_d^1\): Suspect 2 did not commit murder.

Fact-in-issue 2: whether or not Suspect 2 fired the gun in question

• \(H_p^2\): Suspect 2 fired the gun,
• \(H_d^2\): Suspect 2 did not fire the gun.

Fact-in-issue 3: whether or not the glass fragments found on Suspect 2’s clothing originated from the smashed window of the deceased’s house

• \(H_p^3\): The glass fragments originate from the smashed window,
• \(H_d^3\): The glass fragments originate from a source other than the smashed window.

In this example, we have extracted the information from the case scenario, formulated potential facts-in-issue, and then created potential competing propositions. The propositions isolate individual issues that appear in the case scenario and make a clear statement about the competing assertions for each issue. Using this process reduces complex case scenarios into individual unambiguous points of focus. This provides an anchor around which the evidence can be assessed.

For case scenarios such as this, some of the information is potentially irrelevant to the experts’ task and there is the potential to introduce cognitive bias. Expert’s are trained to mitigate this risk and to follow standard operating procedure’s to mitigate this risk by design.