Gauging the Value of Evidence – Crime Scene Investigation
It is unique – If an item is found that helps narrow the possibilities of who might be considered a suspect, or the manner in which a crime was committed, this evidence would be of use. Is an impression from a vehicle tire found in the dirt at the scene? The tread impression can be compared to others to determine the type of tire that was on the car. Is a shoe print left in the soil? The tread may help to identify the size and type of shoes it came from and the wear pattern could be used to match it to a specific pair.
It has a low probability of occurring by chance – Considering the mathematical probabilities will help to determine the odds that a piece of physical evidence found at the scene could appear merely by coincidence. If DNA evidence found at the scene matches a suspect, the chances are exceedingly low that another person can have left this sample. But even evidence that has a much higher probability – for instance, a common type of shoeprint that is left in the soil – is still valuable. When combined with other high probability evidence, these can help narrow the list of possible parties and build a compelling case.
It is consistent – If an item is found that is out of place or inconsistent with the settings, or is out of character for the victim – for instance id he victim was a non-smoker but a cigarette butt is found at the scene – this could be an important bit of evidence.
It is a physical match – If trace evidence is found on the suspect or in his possession that matches something at the scene, this makes this item valuable as evidence. For instance, broken plastic parts or a broken fingernail that can be matched by fracture marks can demonstrate that two pieces were once a part of the same item.