Impression evidence can be gathered from any patterned undersell and is not confined to footwear. The tyres of a vehicle can leave markings that are of use to the criminal justice system in the same way that footwear can be. By far the most common marking to be relied upon is a footprint found in a surface like mud or paint. Impression evidence involves no more than the view of an expert upon the similarities between the mark found at the scene and any seized footwear for pattern size, wear and damage.
A mark that bears the same patter and size as a particular shoe does not identify the wearer conclusively. It does shoe that a person wearing a shoe of that kind and size was at the location. That person may have been the suspect or one of many others. National distribution figures are likely to reveal thousands of sales of that kind of footwear and they only consider the volume of brand names legitimately sold. There is no record of the number of counterfeits in circulation. These statistics are misleading anyway because a particular shoe may be common in an area because a local retailer has brought a large number or discounted the price or just because of local fashions.
However, a likeness in wear and damage is compelling. The damage to our shoes is caused by random events. The wear is caused by gait. Gait analysis is the subject of expert evidence which is beyond the scope of this article. However, a footprint expert can comment upon the wear to a shoe and offer a view upon whether it is consistent with the gait of the suspect. Impressions left upon a surface that show signs of identical damage and wear to the footwear under scrutiny suggest that only a person wearing those shoes could have left the print in question.
Such incontrovertible markings are rare. They can only be made by a shoe that has deteriorated by regular use. A mark from a new shoe gives the evidence gatherer no more than a pattern and size comparison. Further, it relies upon expeditious seizure of the footwear. Shoes worn regularly after a print was made will sustain more damage. Experts can still make comparisons and will consider the possibility that damage was caused by further use but it weakens the evidence. An expert could no longer say categorically that was the shoe that made the mark at the scene. Also, there may be temporary damage upon the print which has subsequently worn away from the shoe such as a small stone that had become lodged in the undersell but has been released. Impression evidence is at it’s most useful to the party that relies upon it if it is taken from a used shoe that was seized immediately.
A defence should not be abandoned because an expert offers the view that a marking could only have been made by a particular shoe. Expert evidence is capable of challenge like any other. An expert witness offers only his opinion. However distinguished an expert is, he can always be wrong. The case of R v T  EWCA Crim 2439 should be considered. Before this appeal experts would express their conclusions to a jury by a ratio calculated in this way. The similarities in pattern, size, wear and damage were ascribed a ratio each which were added to arrive at an overall number. That number was divided by four to give the overall ratio. In the case of R v T this practice was criticised as a mathematical calculation is only as good as the data relied upon. Footprint evidence was specifically differentiated from DNA evidence because a person’s DNA is not affected by regional factors and distribution statistics that cannot be ascertained. The outcome of this case is that, while an expert can still express his opinion upon his findings, he should not do so in the form of a ratio. Another expert may form a different opinion and then the evidence is more balanced.
Impression evidence is valuable to the Crown. Even where only a pattern and size match is available it can still be used to support other evidence. If the markings are unique then it can be used to point compellingly to one particular defendant. There are challenges that may be of use to the defence but the merit of footprint evidence should be considered.
Jo Morris, barrister
0207 936 3637