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July 2009 - Volume 11 - Number 3

 

The Forensic Analysis of Footwear Impression Evidence

Michael B. Smith
Document Analyst/Forensic Examiner
Questioned Documents Unit
FBI Laboratory
Quantico, Virginia

Introduction | Validation | Conclusion | Acknowledgment | References

Introduction

The basis for footwear impression evidence is determining the source of a footwear impression recovered from a crime scene. The process of examining footwear impression evidence takes into account class and identifying characteristics. Class characteristics are those characteristics that result from the manufacturing process, such as physical size, design, and mold characteristics. In contrast, identifying characteristics do not result from the manufacturing process, but are accidental, unpredictable characteristics that result from wear. Identifying characteristics include objects that have become attached to the outsole—such as rocks, thumb tacks, or tape—or marks on the outsole caused by cuts, nicks, gouges, and scratches.

An examiner first determines whether a correspondence of class characteristics exists between the questioned footwear impression and the known shoe. If the examiner deems that there are no inconsistencies in class characteristics, then the examination progresses to any identifying characteristics in the questioned impression. The examiner compares these characteristics with any identifying characteristics observed on the known shoe. Although unpredictable in their occurrence, the size, shape, and position of these characteristics have a low probability of recurrence in the same manner on a different shoe. Thus, combined with class characteristics, even one identifying characteristic is extremely powerful evidence to support a conclusion of identification. According to William J. Bodziak (2000), “Positive identifications may be made with as few as one random identifying characteristic, but only if that characteristic is confirmable; has sufficient definition, clarity, and features; is in the same location and orientation on the shoe outsole; and in the opinion of an experienced examiner, would not occur again on another shoe.”

In the majority of footwear examinations resulting in an association between a questioned footwear impression and a known shoe, the conclusion most often reached is the correspondence of combined class characteristics such as design and physical size. An association based on class characteristics conveys that the known shoe “could have made” the questioned footwear impression, but another shoe having the same design and physical size also could have made the questioned impression. It should be noted that design and physical size can be used independently to eliminate a shoe as a source of an impression.  

The correspondence of combined class characteristics is valuable information, and its importance should not be understated. Shoes come with thousands of different outsole designs and numerous sizes for each design. Thus even a popular shoe outsole design represents only a small subset of all manufactured shoes. Because the subset of shoes that has a particular set of class characteristics is small, class characteristics are very powerful for eliminating shoes as the source of an impression. Based on information provided by footwear manufacturers regarding shoe designs and size ranges, the estimated frequency of any particular shoe design in a specific size is much less than 1 percent of the total shoe population (Bodziak 2000). Although training and experience provide an understanding of the general occurrence of combined class characteristics, footwear examiners do not specify a percentage or likelihood that a shoe of a particular design and size made an impression. Precise quantitative assessments are not provided because accurate information is lacking regarding the exact number of shoes produced in a particular design, size, and geographic distribution, as well as how many shoes of that design and size remain in use.

Additionally, valuable information in the manufacturing process can further narrow an outsole to a specific mold. For example, a particular manufacturer may use 8 to 12 different molds to produce the same outsole design for the same model and size of men’s shoe; however, each of these molds may be different (Bodziak 2000). Mold variations may include design variations, stippling or other texturing, and positioning of the logo (Bodziak 2000). These mold variations make it possible to associate a particular shoe outsole with a particular mold to the exclusion of other molds. By identifying a particular mold, the footwear examiner may conclude that the questioned footwear impression corresponds in design, physical size, and mold characteristics with the known shoe, thus reducing the potential population of shoes of this design and size that could have made the impression. The three outsole designs depicted in Figure 1  show the same general outsole pattern, but upon closer examination in Figure 1.1, each outsole was manufactured using a different mold.

Figure 1

Figure 1 shows Three Nike Air Force I outsoles with the same general outsole design feature.


Figure 1.1

Figure 1.1 shows the enlarged areas of the same outsoles depicted in Figure 1. Three red arrows on each outsole depict the mold variations in each of the different molds, which are along the outside of the toe portion of the shoe.

Occasionally, examiners contact footwear manufacturers for information regarding a particular mold and other distribution information for a particular brand and model shoe. Because not all manufacturers maintain these records, this information may or may not be available. If this information is available, the manufacturer may be able to trace the outsole to a certain mold as well as provide the number of shoes manufactured in that mold, the year the shoe was manufactured, the stores where the shoe was shipped, and the number of shoes and sizes shipped to a particular store. If available, this data can support and strengthen the significance and weight of the association rendered by the examiner regarding the rarity of combined class characteristics.

Footwear impression evidence left behind at crime scenes may provide valuable investigative information even when suspects have not been apprehended yet. The FBI Laboratory maintains a footwear database that is a computerized reference collection of more than 14,000 shoe outsoles from hundreds of different footwear manufacturers. Many police departments and crime laboratories throughout the United States use similar computerized footwear databases. Examiners search questioned footwear impressions through these databases to determine the brand and/or model name of the footwear that left an impression at the crime scene. This investigative information may potentially lead to the suspect of the particular crime.

If identifying characteristics are present in the questioned footwear impression, the examiner compares the same areas of the known shoe to determine if the size, shape, position, and orientation of these characteristics correspond. In order for the examiner to make a positive identification, these characteristics must have sufficient clarity and be in the same location and orientation in the questioned impression and on the outsole of the known shoe. If identifying characteristics are present in the questioned impression and in the corresponding location on the known shoe, a positive identification can be made.

The converse must be interpreted with caution. If identifying characteristics appear on the shoe outsole but not in the questioned footwear impression, they could have occurred after the crime or they did not reproduce in the crime scene impression.

The identifying characteristic need not be sharp in detail but must be clear enough to allow for a determination of the shape, size, and position of the characteristic. The images depicted in Figure 2 show the identifying characteristics in the questioned impression (A) and the corresponding characteristics on the known shoe (B).

example foot impressions
Figure 2 shows the identifying characteristics observed in the questioned impression (A), on the left, and the corresponding characteristics observed on the outsole of the known shoe (B), on the right. The five red arrows indicate the position of the identifying characteristics, which are in the toe portion and arch area of the shoe.

Currently, footwear examiners qualitatively communicate their results in court by describing the common features they have observed. Some researchers (Stone 2006) have developed mathematical models to predict the frequency of accidental characteristics. These models support the extreme rarity of such features.

Validation

In footwear impression evidence, the significance of a positive identification can be conveyed better by “quantification.” The identifying characteristics observed in the questioned impression and the corresponding characteristics observed on the outsole of the known shoe can be quantified by using the position, size, and orientation of each identifying characteristic. An association between a questioned footwear impression and a known shoe, on the other hand, relies on a combination of the outsole design and the physical size of the outsole. These areas of comparison are considered “class characteristics,” and the frequency of occurrence of these class characteristics is based on the number of shoes manufactured in a particular design and size. Often, class characteristics have been mistakenly regarded as having little or no value. However, any shoe outsole in a specific size represents a very small portion of all of the shoes manufactured in that design.

According to the World Footwear Markets 2007 report, in 2005 a world population of 6.6 billion people bought more than 13 billion pairs of shoes (as cited in “Footwear in the Global Economy” [Hereafter “Footwear”] 2007). In 2006, 2.4 billion pairs of shoes were purchased in the United States (as cited in “Footwear” 2007). More than 6,000 different footwear brands are sold in the United States, with more than 600 new brands introduced each year (as cited in “Footwear” 2007). Therefore, associating the suspect’s shoe with the same outsole design as a footwear impression recovered from a crime scene does have notable quantifiable value.

One of the most popular shoes sold in the United States today is the Nike “Air Force I.” Nike first introduced the Air Force I in 1982, and 27 years later, it remains one of the most popular shoes on the market today. In fact, Nike has sold more than 33 million pairs of Air Force I shoes for men. More than 1,000 different molds are available for producing the Air Force I in men’s sizes ranging from size 6 through 21, including half sizes.

Table 1 shows the number of molds for each shoe size. The fewest number of shoe molds for a shoe size is 2 (size 21) and the greatest number of molds is 94 (size 10). The more molds there are, the more each size can be divided into subcategories. Each mold has its own unique design features that are imparted onto each outsole manufactured in it.

Table 1: Nike Air Force I Molds (Men’s)

Shoe Size
Number of molds
Shoe Size
Number of molds
Size 6
89
Size 12
60
Size 6 1/2
69
Size 12 1/2
13
Size 7
78
Size 13
42
Size 7 1/2
49
Size 13 1/2
4
Size 8
64
Size 15
14
Size 8 1/2
75
Size 16
13
Size 9
87
Size 17
12
Size 9 1/2
90
Size 18
12
Size 10
94
Size 19
2
Size 10 1/2
83
Size 20
3
Size 11
77
Size 21
2
Size 11 1/2
40
 
 

Every shoe outsole manufactured from a specific mold exhibits the characteristics of that mold. Because variations exist from mold to mold, outsoles can be subdivided by molds. These variations can be observed in various stippling patterns, concentric circles, vertical bars, and the position of the logo box (Hamm 1989). The outsoles depicted in Figure 1 show some of the mold variations that exist from mold to mold. Although mold characteristics are “class characteristics,” they should be examined closely during the examination process. If variations in the mold characteristics are observed, they can be used to eliminate footwear impressions that share the same general outsole design and physical size.

Although each mold used in making an Air Force I shoe is unique from every other mold, some common design features are present in every mold. The most common design features are the concentric circles in the toe and heel portions of the outsole of the shoe. The top portion of the toe area contains a series of star-shaped designs that are hand-applied to the mold. These star patterns as well as how the concentric circles intersect with the series of short bars along the outside edge are some of the features that can vary from mold to mold.

During the examination process, the examiner also must consider the amount of wear observed on the outsole of the shoe. Wear can be defined as the erosion of the design features or design elements on the outsole of the shoe. This erosion occurs when the outsole of the shoe comes in repeated contact with a particular surface. Cassidy (1980) studied the duplication of wear characteristics and concluded that the more an article of footwear is worn, the less likely general wear will be duplicated from one shoe to the next. Cassidy also found that general wear should not be used as a basis for identification but that the value of the wear becomes greater the more the footwear is worn. Just as design and physical size are, wear can be used to identify or eliminate a shoe as a potential source of an impression. For instance, if more wear is observed in a footwear impression than on a known shoe, then that shoe should be eliminated as the source of that impression, even if the design and physical size correspond.

Wear features can be used as identifying characteristics in two instances (Bodziak 1999). First, when shoes repeatedly come in contact with a surface, an abrasion pattern starts to form on the outsole of the shoe. This abrasion pattern is referred to as a Schallamach (Schallamach 1952) pattern, or feathering (Tart et al. 1996). The pattern is named for Adolph Schallamach, who first discovered this pattern on tires.

The Schallamach pattern is a ridge-like design, similar to a fingerprint, that appears on the flat part of the shoe outsole. This pattern changes within 48 to 50 hours of continuous wear (Tart et al. 1996). Davis and Keeley (1997) conducted additional research and concluded the following regarding the Schallamach pattern:

  • Of the 258 footwear identifications made by the Metropolitan Police Forensic Science Laboratory from 1991 to 1994, 28 involved the Schallamach pattern.
  • The Schallamach pattern was observed mainly in the toe and heel area of the outsole, ran horizontally across the outsole of the shoe, and was not affected by the design of the outsole.
  • The ridge spacing of the Schallamach pattern varied from 0.05 to 0.5 mm.
  • Schallamach patterns were observed on two of the outsoles after less than 9 hours of wear.
  • The Schallamach patterns started to change completely after 6 hours of wear on one outsole and 16 hours on another outsole.
  • The Schallamach patterns observed from shoe to shoe were distinct. (Davis and Keeley 1997)

Thus if this pattern is observed in the crime scene impression and is also observed on the outsole of the suspect’s shoe, provided that the shoes are recovered within a short time frame, strong evidence exists to support a positive identification. The images in Figure 3 depict a Schallamach pattern recovered from a questioned footwear impression (A) and the corresponding ridge pattern and ridge spacing on a test impression made by the known shoe (B).

Figure 3aFigure 3b
Figure 3(A):
Questioned Footwear Impression
Figure 3(B):
Test Impression from Known Shoe  

Figure 3: The arrows in the lower right-hand corners in the figures above depict the ridge spacing of the Schallamach pattern in the questioned footwear impression (3A) and the corresponding ridge spacing on the outsole of the known shoe (3B).

Wear also can be used as an identifying characteristic when the wear observed on the outsole of the shoe becomes so severe that jagged edges or holes develop. Identifying characteristics can be defined as objects such as rocks, thumbtacks, pieces of glass, and other items that have become attached to the outsole of a shoe during wear and have left such marks as scratches, cuts, or nicks. Identifying characteristics observed in the crime scene impression and on the outsole may allow the footwear examiner to make a positive identification.

Cassidy’s research (1995) on the duplication of individual or accidental characteristics also found that the duplication of a single characteristic in a general area can occur, albeit infrequently, but the duplication of multiple individual characteristics in identical areas does not occur. He also found that an identification of an impression with an article of footwear is possible. Cassidy also determined that the duration of individual characteristics depends on the type of outsole material. For example, leather is a harder material and therefore undergoes changes due to wear and tear more rapidly than a rubber outsole.

As stated earlier, Stone (2006) developed a hypothetical probability model for identifying characteristics. A metric grid with divisions of 1 mm2 was used to help quantify the occurrence of identifying characteristics on a shoe outsole. By placing the grid over a size 8 1/2 shoe, Stone created a 16,000 mm2 grid on the surface area of the shoe outsole. Stone used the following simple formula to calculate the probability of a single characteristic (point):

P subscript e equals m divided by n.

where Pe = probability of an event occurring,

m = number of ways of success, and

n = total number of possible outcomes.

The probability that another outsole contains a single characteristic (point) in the same position is 1 in 16,000. This probability can be expressed by the following formula:

For 1 point characteristic (pc),

P subscript 1pc (point characteristic) equals 1 divided by 16,000 equals .000625 equals 1 out of 16,000.Later

When two or more identifying characteristics (points) are observed in an outsole, then the following formula can be used to calculate the probability of occurrence:

Subscript n, C, subscript r equals the factorial of n divided by the factorial of n minus r multiplied by the factorial of r. The factorial of a number equals the number times all of the positive whole numbers less than it. For example, the factorial of 5 is 5 times 4 times 3 times 2 times 1 equals 120.

where nCr = the combination of r items taken n at a time,

n = the number of possible locations on the outsole, and

r = the number of characteristics/points. (Stone 2006)

The following chart demonstrates the probability that two shoe outsoles will contain identifying characteristics in the same location, based on the number of characteristics.

Number of Characteristics Probability of Occurrence
1 1 out of 16,000
2 1 out of 127,992,000
3 1 out of 683 billion
4 1 out of 2.7 quadrillion
5 1 out of 8.7 quintillion
6 1 out of 23 sextillion
7 1 out of 53 septillion
8 1 out of 106 octillion
9 1 out of 189 nonillion
10 1 out of 300 decillion
Source: Stone 2006

This study does make some assumptions, for example, that all grids are independent and that the exact number of grids will change as the shoe size increases or decreases. However, in concert with the powerful class characteristics, it is highly unlikely that two shoes would share the same class and identifying characteristics. Although the preceding statistical formulas are not commonly used in rendering a conclusion in day-to-day footwear examinations, they do offer some valuable insight into the uniqueness of the combination of class and identifying characteristics observed on the shoe outsole.

Another study attempted to quantify the significance of individualizing characteristics. The Mount Bierstadt Study (Adair et al. 2007) involved the use of 12 pairs of Hi-Tec Altitude II hiking boots. The six participants (three men and three women) were each provided a pair of unworn boots at the beginning of the climb and a second pair of unworn boots on the way down. The elevation of Mount Bierstadt is 14,065 feet, and the participants hiked about 3.5 miles in each direction, for a total hiking time of 3.5 hours.

At the completion of the hike, the boots were examined for the presence of identifying characteristics. The boots were compared visually to determine if sufficient detail was present to individualize each boot. Each boot did contain a sufficient number of identifying characteristics necessary to enable distinction of one boot from another. Transparent test impressions were then made of each boot and compared with every other boot.

The men’s and women’s boots differed somewhat in the number of outsole design elements containing at least one identifying characteristic. The men’s boots contained identifying characteristics on 44 percent of the design elements, whereas the women’s boots contained characteristics on only 33 percent of the outsole design elements. Although a small study, it does support the hypothesis that accidental characteristics acquired on the outsoles of shoes are rare and can be used to individualize.

The value of each individual and accidental characteristic on an outsole depends on an experienced evaluation of the traits and attributes of that characteristic. An area of damage is examined for shape, size, relative location on the outsole, and any other distinctive characteristics. All of these traits add to or subtract from the value of an individual characteristic. For instance, a pin-sized, circular hole would be less valuable than a jagged tear in a tread element. Because damage is random and unpredictable, the value of each characteristic that may be present on an outsole is unpredictable. Also, the uncontrollable nature of the size and quality of a crime scene impression makes the reproduction of any individual characteristic random. Because all of these factors contribute to the unpredictable nature of the information provided in a crime scene impression, it is not possible to define a minimum number of characteristics needed to accurately identify an impression with the source outsole.

In 2004 the FBI created the Scientific Working Group on Shoeprint and Tire Tread Evidence (SWGTREAD) to serve as a professional forum in which forensic experts in the area of shoeprint and tire tread impression evidence share, discuss, and evaluate methods, techniques, protocols, quality assurance, educational requirements, and research relating to shoeprint and tire tread evidence. SWGTREAD comprises shoeprint and tire tread examiners from government laboratories and private industry throughout the United States. SWGTREAD has since written and published guides for shoeprint and tire tread examiners (SWGTREAD 2005a–f, 2006a–d).

The SWGTREAD guidelines are the first major attempt by a professional forum of shoeprint and tire tread evidence examiners to establish consensus in the detection, collection, and analysis of shoeprint and tire tread evidence. These published guidelines are only recommendations, not mandates. They serve to communicate this “best practice” consensus to practitioners in the field.

Conclusion

Because footwear impressions are found at virtually every crime scene, footwear impression evidence often provides an important link between the suspect and the crime scene. As a significant form of physical evidence, impressions left behind at the crime scene may provide valuable information on where the crime occurred and the direction the suspect traveled while committing the crime. This information may place the suspect at the crime scene or eliminate the suspect as having been there.

Although interest in footwear evidence has increased over the years, many crime scene investigators and crime scene technicians still fail to recognize the importance and value of footwear impressions as physical evidence. Often, impressions are overlooked, improperly collected, or not collected at all. However, with the increased awareness of footwear impression evidence, accompanied by continued research and additional training now being offered on the proper detection, recovery, and collection of footwear evidence, the analysis of this valuable form of physical evidence will continue to be an integral part of criminal investigations.

Acknowledgment

This is publication number 08-22 of the Laboratory Division of the Federal Bureau of Investigation. Names of commercial manufacturers are provided for identification only, and inclusion does not imply endorsement by the FBI.

 References/Suggested Readings

Adair, T. W., Lemay, J., McDonald, A., Shaw, R., and Tewes, R. The Mount Bierstadt Study: An experiment in unique damage formation in footwear, Journal of Forensic Identification (2007) 57:199–205.

Bodziak W. J. Footwear Impression Evidence: Detection, Recovery, and Examination. 2nd ed. CRC Press-Taylor & Francis, Boca Raton, Florida, 2000, pp. 334, 347, 352, 413.

Cassidy, M. J. Footwear Identification. Lightning Powder Company, Salem, Oregon, 1995.

 Davis, R. J. and Keeley, A. Feathering of footwear, Science & Justice (2000) 40:273–276.

Footwear in the global economy, World Footwear, September/October 2007, p. 9.

Hamm , E. D. The individuality of class characteristics in Converse All-Star footwear, Journal of Forensic Identification (1989) 39(5):277–292.

Schallamach, A. Abrasion of rubber by a needle, Journal of Polymer Science (1952) 9(5):385–404.

Schallamach, A. Abrasion pattern on rubber, Transactions of the Institution of the Rubber Industry (1952) 28:256–267.

Schallamach, A. Friction and abrasion of rubber, Wear (1957–1958) 1:384–417.

Schallamach, A. A theory of dynamic rubber friction, Wear (1963) 6:375–382.

Schallamach, A. Abrasion, fatigue, and smearing of rubber, Journal of Applied Polymer Science (1968) 12:281–293.

Schallamach, A. How does rubber slide? Wear (1971) 17:301–312.

Stone, R. S. Footwear examinations: Mathematical probabilities of theoretical individual characteristics, Journal of Forensic Identification (2006) 56:577–599.

SWGTREAD. Scope of work relating to forensic footwear and/or tire tread examiners, Journal of Forensic Identification (2005a) 55:764–765.

SWGTREAD. Guide for the detection of footwear and tire impressions in the field, Journal of Forensic Identification (2005b) 55:766–769.

SWGTREAD. Guide for the collection of footwear and tire impressions in the field, Journal of Forensic Identification (2005c) 55:770–773.

SWGTREAD. Guide for the detection of footwear and tire impressions in the laboratory, Journal of Forensic Identification (2005d) 55:774–777.

SWGTREAD. Guide for the collection of footwear and tire impressions in the laboratory, Journal of Forensic Identification (2005e) 55:778–780.

SWGTREAD. Guide for the preparation of test impressions from footwear and tires, Journal of Forensic Identification (2005f) 55:781–786.

SWGTREAD. Guide for minimum qualifications and training for a forensic footwear and/or tire tread examiner, Journal of Forensic Identification (2006a) 56:788–793.

SWGTREAD. Guide for the forensic documentation and photography of footwear and tire impressions at the crime scene, Journal of Forensic Identification (2006b) 56:794–799.

SWGTREAD. Guide for the examination of footwear and tire impression evidence, Journal of Forensic Identification (2006c) 56:800–805.

SWGTREAD. Standard terminology for expressing conclusions of forensic footwear and tire impression examinations, Journal of Forensic Identification (2006d) 56:806–808.

Tart, M. S., Downey, A. J., Goodyear, J. G., and Adams, J. Unpublished report. The Appearance and Duration of Feathering As a Feature of Wear. FSS Report No. RR 786. Forensic Science Service, Birmingham, England, August 1996, 1–11.