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Research and Technology - Forensic Science Communications - July 2007

Research and Technology - Forensic Science Communications - July 2007

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July 2007 - Volume 9 - Number 3

Research and Technology

A Validation Study for Duct Tape End Matches1

Maureen J. Bradley
Supervisory Chemist/Forensic Examiner
Chemistry Unit
FBI Laboratory
Quantico, Virginia

Roger L. Keagy
Chemist
Chemistry Unit
FBI Laboratory
Quantico, Virginia

Preston C. Lowe
Chemist/Forensic Examiner
Chemistry Unit
FBI Laboratory
Quantico, Virginia

Michael P. Rickenbach
Chemist/Forensic Examiner
Chemical-Biological Sciences Unit
FBI Laboratory
Quantico, Virginia

Diana M. Wright
Chemist/Forensic Examiner
Chemistry Unit
FBI Laboratory
Quantico, Virginia

Marc A. LeBeau
Unit Chief
Chemistry Unit
FBI Laboratory
Quantico, Virginia

Abstract | Introduction | Materials and Methods | Results | Discussion | Acknowledgments | References

Abstract

Fracture matches are considered to be the strongest association achievable in forensic examinations of glass, metal, wood, plastic, paint, tape, and other trace evidence. Despite the fact that these examinations are fairly routine, few publications exist to support their admissibility in court. This study was designed, using duct tape as the fractured medium, to determine the validity and error rate associated with conducting end-match (fracture-match) examinations on this material. Five test designs, which varied either the source roll of tape or manner of separation (torn or cut) from the roll, were administered to four analysts with instructions to examine the assigned test sets for end matches. If an end match was not identified by the initial analyst, the entire test set was independently evaluated by the remaining three analysts. Results indicated that while tape grade did not hinder end-match identification, the manner of separation could affect results.

Introduction

Forensic laboratories are frequently tasked with comparing tape specimens to establish a possible evidentiary link between a suspect and a particular crime or between different crimes. Tape associated with the commission of a crime may have been used as a gag or bindings, to seal packages or threat letters, or in the construction of an improvised explosive device. A forensic tape examination may involve either (1) an analysis to identify or characterize the tape components to possibly determine the manufacturer and product grade or (2) a comparative analysis between two (or more) specimens to determine if they share a common origin. These determinations can be made by sourcing distinctive features of the tape, conducting a physical reconstruction of tape pieces, and/or conducting a comprehensive chemical analysis of the tape components.

Although tape end-match examinations have been conducted in forensic science laboratories for decades (Agron and Schecter 1986; Blackledge 1987; Cortner and Hamman 1996; Courtney 1994; Kirk 1953; Osterburg 1967), there are no published validation studies for this technique. In the era of Frye and Daubert where a forensic analyst must demonstrate to the court the scientific soundness of his or her examinations, the FBI Laboratory’s Chemistry Unit undertook a study to address this issue.

This study was designed to determine the validity of conducting end-match examinations on duct tape evidence and to evaluate the error rate associated with such an examination. This study was also designed to address the following: the number of end matches identified correctly and incorrectly, the ability to identify end matches for both torn and cut pieces of duct tape, and whether the grade of tape influences the ability to identify end matches. Duct tape was chosen because it is the most frequently encountered tape evidence in our laboratory.

Materials and Methods

Three rolls of duct tape, representing different manufacturers and grades of tape, were used in this study. Tape products are formulated and marketed for a specific end use. As a particular manufacturer’s product grade increases from economy or utility grade to industrial, contractor, and premium grades, so too do such properties as adhesion, overall thickness, yarn count, and price. Two utility grades and one industrial grade of duct tape were selected for this study and purchased at home-improvement stores. Based on the researchers’ experience, these tapes are representative of the types of duct tape typically encountered in forensic examinations. The specifications for each roll of tape are shown in Figures 1–3.

Figure 1: Utility-grade duct tape, Nashua 300 (Tyco Adhesives)

Figure 2: Utility-grade duct tape, MANCO, Inc. (Henkel Consumer Adhesives, Inc.)

Figure 3: Industrial-grade duct tape, Nashua 398 (Tyco Adhesives)

Five tests were designed that varied the roll of tape and/or the mode of separation from the roll. The test designs were designated as 1 through 5. Three contained torn ends prepared from each of the three master rolls of duct tape, and two contained scissor-cut ends prepared from two of the master rolls of different grades of duct tape. Four sets, designated as A through D, of each test design were prepared for administration to the four test participants. Each of the 20 test sets was prepared as follows: 10 strips of tape were either cut or torn from the master roll and adhered sequentially to a plastic substrate (Kapak [Kapak Corporation, Minneapolis, Minnesota]). Sequential numbers (1–10) were marked on the plastic substrate adjacent to the individual strips. Each strip of tape was randomly labeled alphabetically in a manner that did not indicate its correct orientation. Each test set was digitally photographed to document the original sequential order of the tape strips. The order within each test set was also verified by a second person. The pieces of tape were then separated from one another by cutting the plastic substrate. The substrate was cut in such a way as to remove the sequential number and to prevent end matching of the substrate. For each test set, the 10 strips of tape were shuffled together, and 3 strips were removed at random. The 3 randomly removed strips were placed into a labeled plastic bag. The 7 remaining strips were placed into individual plastic bags and labeled appropriately. Depending on which of the 7 strips remained from the original 10, the number of end matches per test set could vary from 3 to 6.

Each of the four analysts received five test sets, one of each test design (e.g., 1A, 2B, 3D, 4B, 5C). They were instructed to evaluate whether end matches existed among the seven strips of tape in each set following the FBI Laboratory’s standard operating procedure for determining end matches of tape.

Results

The Association of Firearms and Tool Mark Examiners (AFTE) defines a physical match (fracture match) as “the examination of two or more objects either through physical, optical or photographic means which permits one to conclude whether the objects were either one entity or were held or bonded together in a unique arrangement” (AFTE 1994). End matches between two torn pieces of duct tape can be established based on both the alignment of the tear in the backings, as well as the alignment of the severed yarns comprising the fabric reinforcement along the margin of the tear (Figure 4). A mild solvent, such as hexane, may be used to remove enough of the adhesive layer to expose the reinforcing yarns. With an end match, the complementary pair of warp (machine direction) yarns will line up along the fracture. The fill (cross direction) yarns may also be severed at some point along the fracture.

Figure 4: Test Set 1C, a torn duct tape end match viewed from the backing (left) and adhesive (right) sides. Note: In the photograph on the right, the adhesive is partially removed to expose the fabric reinforcement. (The association between pieces designated as C and D is coincidental. The tape pieces were labeled alphabetically at random.)

The same principles apply when evaluating cut ends. However, because of the mode of separation, significantly fewer distinguishing features are observed with cut ends as compared to torn ends. The closer the cut is to 90 degrees, the fewer points of comparison exist to corroborate an end match between two pieces. Viewing the cut end match using transmitted, rather than reflected, light provides an alternative means of evaluating and documenting these matches (Figure 5).

Figure 5: Test Set 4B, a cut duct tape end match viewed with transmitted light

In the initial study, 46 of the 50 hand-torn end matches and 25 of the 31 scissor-cut end matches were identified. The remaining 4 hand-torn end matches and 6 scissor-cut end matches were not identified. It should be noted that there is a distinction between not identified and misidentified. End matches that are not identified are considered inconclusive rather than incorrect. No errors regarding a misidentification of an end match were made in the initial study. That is, none of the analysts incorrectly associated two pieces of tape that did not truly have an end match.

The results of the initial administration of the test are summarized below and in Table 1.

Table 1: Results of validation study for duct tape end matches

Hand-Torn Sets

Test Sets 1A–1D: Nashua (Nashua Tape Products, Tyco Adhesives, Norwood, Massachusetts) 398—Industrial Grade

Eighteen of the 18 existing end matches were identified.

Test Sets 3A–3D: Nashua 300—Utility Grade

Fifteen of the 18 existing end matches were identified.

Test Sets 5A–5D: Manco (Manco, Inc., Avon, Ohio)—Utility Grade

Thirteen of the 14 existing end matches were identified.

Scissor-Cut Sets

Test Sets 2A–2D: Nashua 398—Industrial Grade

Fourteen of the 16 existing end matches were identified.

Test Sets 4A–4D: Nashua 300—Utility Grade

Eleven of the 15 existing end matches were identified.

In test sets where an end match was not identified by the original analyst, the set was reevaluated independently by the three remaining analysts, and each rendered his or her opinion. During this reevaluation process, the remaining three analysts were not informed which of the end matches were not identified by the original analyst. The results of the reevaluation of the test sets are summarized below.

Hand-Torn Sets

Test Sets 1A–1D: Nashua 398—Industrial Grade

Reevaluation results: No test sets were reevaluated because all end matches were identified in the original administration of these test sets.

Test Sets 3A–3D: Nashua 300—Utility Grade

Test Set 3A: Two end matches were not identified in the original administration of the test set.

Reevaluation results of Test Set 3A: All three of the analysts reevaluating this test set identified one of the previously unidentified end matches, but only one of the three analysts identified the second previously unidentified end match.

Test Set 3B: One end match was not identified in the original administration of the test set.

Reevaluation results of Test Set 3B: One of the three analysts reevaluating this test set identified the previously unidentified end match.

Test Sets 5A–5D: Manco—Utility Grade

Test Set 5D: One end match was not identified in the original administration of the test set.

Reevaluation results of Test Set 5D: The three analysts reevaluating this test set correctly identified the previously unidentified end match.

Scissor-Cut Sets

Test Sets 2A–2D: Nashua 398—Industrial Grade

Test Set 2D: Two end matches were not identified in the original administration of the test set.

Reevaluation results of Test Set 2D: The three analysts reevaluating this test set identified the two previously unidentified end matches.

Test Sets 4A–4D: Nashua 300—Utility Grade

Test Set 4A: Three end matches were not identified in the original administration of the test set.

Reevaluation results of Test Set 4A: One of the three analysts reevaluating this test set identified two of the three previously unidentified end matches. The remaining two analysts identified all three of the previously unidentified end matches. However, two misidentifications were also reported during this reevaluation. The misidentifications were by different analysts on different pairs of tape strips.

Test Set 4B: One end match was not identified in the original administration of the test set.

Reevaluation results of Test Set 4B: Two of the three analysts reevaluating this test set identified the previously unidentified end match.

Discussion

This test was designed to assess both the validity of conducting end-match examinations on duct tape and the error rate associated with such an examination. Based on the results of this study, no errors (misidentifications) were found when conducting hand-torn end matches on three different types of duct tape. Ninety-two percent of the end matches that existed were identified. The end matches that were not identified or reported are considered inconclusive rather than false negatives. For a number of these instances, the original analyst and/or the analyst(s) reevaluating the test sets indicated in their written documentation that the unreported end match was possible. However, in their opinion, there were insufficient points of comparison to draw a definitive conclusion. In those cases, the analyst would proceed with the comparison analysis by employing the full analytical scheme defined in the FBI Laboratory’s protocols to determine if the pieces could be associated based on class characteristics (adhesive color, backing texture, fabric weave, yarn count, width, and thickness) and chemical composition of each of the tapes’ components.

Insufficient points of comparison to definitively conclude an end match were more apparent in the scissor-cut-end test sets. The number of end matches identified for the cut ends was lower than for the torn ends (81 percent vs. 92 percent). Although no errors (misidentifications) were reported in the initial administration of the test, reevaluation of the test sets resulted in two misidentifications by different analysts. Based on these findings, it is recommended that an analyst proceed with caution when conducting end matches on cut pieces of duct tape. Jagged-edge cuts or ends resulting after the tape has been bunched up and then cut would have more points of comparison for aligning two pieces than straight or near-straight cuts.
 
Three types of duct tape were included in this study in an attempt to determine if the grade of tape has an effect on the ability of an analyst to successfully conduct these examinations. The fact that there were no errors in determining the end matches that existed among the hand-torn ends of each type of duct tape would suggest that the grade of duct tape does not affect one’s ability to conduct these examinations. However, a larger study would have to be undertaken in order to fully evaluate this variable.

The design, administration, and evaluation employed in this study can be used as a model to evaluate additional duct tapes, other types of tape, or other materials frequently encountered in forensic examinations.

Acknowledgments

1This paper was presented at the American Academy of Forensic Sciences 56th Annual Meeting, Dallas, Texas, February 19, 2004. It was originally published in the May 2006 issue of the Journal of Forensic Sciences. It is reprinted here with the permission of the editor. Minor editorial changes affecting only the grammar, style, and format but not the factual content were made. Individuals wishing to cite the paper should refer to and cite the original:

Bradley, M. J., Keagy, R. L., Lowe, P. C., Rickenbach, M. P., Wright, D. M., and LeBeau, M. A. A validation study for duct tape end matches, Journal of Forensic Sciences (2006) 51:504–508.

This is publication number 04-18 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.

Coauthor Roger L. Keagy retired from the FBI on May 31, 2007.

The authors wish to thank Robert F. Mothershead II of the Chemistry Unit of the FBI Laboratory for his assistance in the preparation of this study.

For further information, please contact:

Maureen J. Bradley
Chemistry Unit
FBI Laboratory
2501 Investigation Parkway, Room 4220
Quantico, Virginia  22135
maureen.bradley@ic.fbi.gov

References

Agron, N. and Schecter, B. Physical comparisons and some characteristics of electrical tape, AFTE Journal (1986) 18(3):53–59.

Blackledge, R. D. Tapes with adhesive backings: Their characterization in the forensic science laboratory. In: Applied Polymer Analysis and Characterization: Recent Developments in Techniques, Instrumentation, Problem Solving. J. Mitchell, Jr., ed. Carl Hanser Verlag, Munich, 1987, pp. 413–421.

Cortner, G. V. and Hamman, J. E. Physical match: A focus on its forensic use and interpretation in criminal cases, TIELINE (1996) 20(1):32–49.

Courtney, M. Evidential examinations of duct tape, SWAFS Journal (1994) 16(1):10–16.

Kirk, P. K. Crime Investigation: Physical Evidence in the Police Laboratory. Interscience Publishers Inc., New York, 1953, pp. 116–125.

Osterburg, J. W. The Crime Laboratory: Case Studies of Scientific Criminal Investigations. Indiana University Press, Bloomington, Indiana, 1967, pp. 96–115.

Association of Firearms and Tool Mark Examiners (AFTE) Standardization Committee. Glossary of the Association of Firearms and Tool Mark Examiners. 3rd ed., Available Business Group, Chicago, 1994.