Determining the Significance of Outsole Wear Characteristics During the Forensic Examination of Footwear Impression Evidence

1. What they did

When a shoe is worn, the outsole (the bottom of the shoe) slowly erodes. Footwear examiners compare this wear in a crime scene print with the wear on a suspect’s shoe. The authors, four forensic footwear examiners, set out to explain how much weight that wear can carry.

They did four things:

• Defined the terms. They set out the standard terms recommended by the Scientific Working Group for Footwear and Tire Tread Evidence (SWGTREAD), such as general wear, degree of wear, position of wear, and class versus individual characteristics.
• Reviewed the literature. They summarized the main books and studies on wear, from 1964 to 2009.
• Listed what can mislead an examiner. Using photographs and test impressions, they showed how time, the surface, and the way a shoe is made can change how wear appears in a print.
• Surveyed examiners. They asked 13 experienced footwear examiners in ten countries five questions about how they judge wear, using pictures of hypothetical cases.

2. What they found

How wear looks

• Most crime scene shoe prints show at least some general wear. Heels usually show wear first, because they strike the ground first with each step.
• Wear depends on how a person stands and walks, how long and on what surfaces the shoe has been worn, the soling material, the wearer’s weight and flexibility, and what the shoe is used for.
• Shoes of the same design worn by the same person for running showed similar forefoot wear. Shoes of the same design worn by different people for work showed similar heel wear. Shoes of one person can also wear differently.

The literature

• Earlier texts (Cassidy 1980; Bodziak 2000) concluded that general wear alone is not enough to identify one particular shoe.
• A study of military boots measured small differences in wear between boots worn under the same conditions. The authors caution that these differences were tenths and hundredths of a centimetre in inked prints made under controlled conditions, and would be of limited use at real crime scenes.
• In a European test (Expert Working Group Marks), most examiners correctly excluded a shoe that was less worn than the shoe that made the prints.
• No study supported identifying a shoe from general wear alone, and neither SWGTREAD guideline on wear allows it.

What can mislead an examiner

• Time: one shoe photographed five months apart showed a large change in wear. Without knowing how long after the crime the shoes were seized, this could lead to wrongly excluding the shoe.
• Surface and deposition: mud or snow can fill the tread, and excess blood can hide detail. Photo distortion, casting or lifting problems, and movement can also blur wear.
• Manufacturing features that look like wear: mold warp in new compression-molded soles leaves gaps in prints on smooth surfaces; low foxing strips and toe or heel guards stop parts of a new sole from touching the ground; some soles are designed to look worn; and in very worn soles a hidden honeycomb or grid inside the sole can show through.
• Pressure: three prints of the same shoe made with different pressure showed the worn area differently.

The survey (13 examiners)

• 11 agreed that general wear is a class characteristic. The two who said no preferred to call it a wear or manufacturing characteristic.
• For a nearly new shoe, 10 said the lack of wear corresponding was worth noting; 3 would not comment on wear.
• For some wear on the outer heel, 11 said it helps narrow down the shoes that could have made the print; 2 thought it not worth comment.
• For wear in a more specific area, all found it significant, but they weighed it differently (4 chose A, 7 chose B, 2 chose C). No one thought it could justify identifying the shoe.
• All 13 agreed that the irregular edges of a tear or hole are individual characteristics.

3. What they concluded

• General wear must always be evaluated in footwear examinations. Even thousands of soles of the same design and size are not all worn in the same places or to the same degree.
• Matching wear helps reduce the number of shoes that could have made a print, and clear differences in wear can exclude a shoe. But general wear alone is not enough to identify one particular shoe.
• Holes, tears and other damage from heavy wear are different: they are individual characteristics that can help identify a shoe.
• Examiners need to weigh the time since the crime, the impression-making process, manufacturing features and anything else that could affect how wear appears in the print.
• Wear used to support a conclusion must be clearly visible, confirmable, and described in terms that show its value.
• The survey suggests this is how the international community of footwear examiners already works. The authors note the survey answers might have agreed more closely had the examiners been able to make their own test prints with the shoe, rather than seeing a single test impression.

4. How tracking was or was not used

This is a forensic paper about reading human shoe prints, not a field tracking study. It deals with tracks and signs identification in a narrow sense: deciding whether a print could have come from a particular shoe, using the shoe’s design, size, wear and damage. It does not involve trailing.

5. Tracker or observer certification

Reported but not validated. The 13 survey respondents were chosen for their experience and their involvement in European footwear meetings. No certification or test of their skill is reported. The authors are practising forensic footwear examiners.

6. Other technologies, skills or innovations

• Test impressions of known shoes compared with photographs of their outsoles.
• Repeat photographs of the same shoe over five months to show how wear progresses.
• A short questionnaire with picture-based hypothetical cases, printed in full in the Appendix.