THE COMPARISON OF EVIDENCE

Definition of terms:

Class characteristics

Characteristics that enable an object to be placed into a particular category, for example identifying a trainer as belonging to a certain brand

Test impression

An impression deliberately made using a suspect item in order to compare it with a scene impression.

Scene impression

An impression detected at the scene of a crime, which may be of potential forensic importance.

Individual characteristics

Characteristics that are unique to a particular object (e.g. a tool, tyre or shoe) and, as such, are potentially useful in the identification of scene impressions

COMPARISON BETWEEN AN EVIDENTIAL OBJECT AND A RELEVANT DATABASE

In some instances, the purpose of this type of comparison is to identify a category to which an item of evidence belongs. To achieve this, the class characteristics of the evidential item concerned are established. For example, if footwear impressions or prints are recovered from a crime scene, their sole patterns may be established and then these may be usefully compared with sole patterns held on a footwear database

Through this exercise, it may be possible to identify the manufacturer and, conceivably, the style of the shoe concerned. This type of footwear comparison is particularly relevant to trainers. Similarly, tyre marks left at an incident scene may be compared with an appropriate database of tread pattern designs.

With some specific types of forensic evidence, namely fingerprints and samples of body fluids or tissues used for DNA profiling, the object of comparison with a database is the identification of the individual concerned.

COMPARISON BETWEEN TWO PIECES OF EVIDENCE OBTAINED FROM DIFFERENT PLACES

This type of comparison seeks to determine whether two pieces of apparently similar forensic evidence, for example hairs, textile fibres, paint chips or glass fragments, may share a common origin. Its purpose, therefore, is to determine whether any possible link exists between the two separate locations from which the evidence has been retrieved. This may be between two individuals (as in the case of the victim of an attack and his or her assailant), between an individual and a crime scene, or even between two different crime scenes. This type of comparison may be usefully illustrated by the following hypothetical scenario.

Consider a case in which a car window is broken and the CD player stolen from the vehicle. A suspect is apprehended by the police and, although the CD player is not in the suspect’s possession, there are splinters of glass adhered to the right-hand cuff of his jacket. A comparison is made between shards of glass taken from the car window and those recovered from the suspect. If these samples are found to be indistinguishable, this provides evidence that is consistent with the suspect being at the crime scene.

COMPARISON BETWEEN QUESTIONED SAMPLES, BOTH POSITIVE AND NEGATIVE CONTROLS, AND REFERENCE COLLECTIONS

A crime scene sample that is to be tested to find its evidential value is usually referred to as a questioned sample (or sometimes a disputed sample). Such tests are designed to evaluate a hypothesis. A hypothesis is a supposition that is either true or false and that can be tested by experimentation. For example, if a suspect is detained and found to possess a packet containing a pale brown powder, then the hypothesis may be that the powder is heroin. In order to test this hypothesis, experiments may be carried out that compare the chemical characteristics of this questioned sample with those of a known sample of heroin. Known samples such as this are referred to as positive controls, known or standards. If the questioned sample and the positive control are shown to have characteristics in common, it might be concluded that the questioned sample is indeed heroin. However, this may not be the case. It is possible that the chemicals and/or equipment used in the test were contaminated with heroin. In order to eliminate this possibility, it is necessary to carry out the test in a way that is identical in all respects to the tests to be carried out on the questioned sample and the positive control sample, except that it contains neither of these materials. Such a test is known as a negative control or a blank. In some instances, it is necessary to go to considerable lengths when carrying out the negative control test. For example, when testing for trace levels of explosives, swabs from all surfaces that will come into contact with the sample will be obtained. These will then be tested to show that the equipment was free from explosives. Note that in many applications, the term ‘control’ is used to denote either positive or negative controls; the context makes it clear which type of control is being referred to. There are circumstances in which it is valuable to compare a questioned sample with a number of positive controls. For example, the properties of a liquid retrieved from a scene of suspected arson may be compared with those of a range of flammable liquids, such as different types of petrol, paraffin and diesel fuel. Through comparison, it may be possible to identify the questioned sample via elimination and positive matching. A collection of positive controls used for such a purpose is known as a reference collection.

COMPARISON BETWEEN A SCENE IMPRESSION AND A TEST IMPRESSION

Impressions made by recognizable objects, such as footwear, tyres and tools, are often detected during the examination of crime scenes. If an object suspected of creating the impression(s) in question is subsequently discovered, then that object may be used to create a series of test impressions. A comparison of these test impressions with the scene impression(s) may reveal that both types were created by objects with the same class characteristics. However, in some cases, it may be possible to proceed beyond this stage and identify the suspect item as being the actual one used in the commission of a crime. This can occur when individual characteristics, namely those that are peculiar to a particular individual object, are shown to be visible on the scene impression(s), as well as on the test impressions. Such individual characteristics may be created by some aberration during the manufacturing process but are more likely to be acquired as a result of general wear and tear. Characteristics that are exhibited in evidence and that are capable of identifying a specific item are said to be individualizing.