Issues in Explosives Residue Analysis A Primer for the Bar Frederic Whitehurst, Ph.D.
[Editor’s Note: This is a multi-part series deigned to educate the defense bar on important issues concerning explosive and explosive residue investigations]
Part 12: Experience: What makes for a proper expert in explosive scene investigation?
Part 13: Conclusion
Probably the most misunderstood field in the area of explosives residue analysis is that of experience. This section will attempt to enlighten the bar as to the jobs each of the players who are called “explosives expert” holds. Generally, when an expert testifies, that expert has a large database of experience to draw from. The individual who analyzes for the presence of drugs is using time-honored protocols and equipment proven again and again to give the correct data. The areas of drug/toxicology analysis and commercial product analysis are prime examples of areas of chemical analysis which have been thoroughly addressed by analytical chemists. When one refers to the literature, volumes upon volumes of information exist describing the analyses of these types of products. They are essentially well defined and very available. The commercial product that comes into question raises very clear issues. Is a material present or not present? Illicit narcotics analysis raises very clear issues. Is a material present or not present? Even when the matrix in which the materials are present is as complex as human tissue, the literature abounds with descriptions of controlled experiments using known conditions on known subjects. Analytical schemes to determine the presence of drugs and their metabolites are well documented. The “experience” in these fields is well documented.
When one wanders into the field of explosives residue analysis, the wandering is akin to Moses travelling out into the desert from an oasis. Immediately the difficulty arises that every criminal bombing that ever occurred or ever will occur happens in a different matrix. Bombs are initiated in planes, in cars, in houses, in soil, in oily environments, in briefcases and radios, and in unlimited numbers of other matrices. And unlike the search for the one product or its well-characterized metabolites, one finds himself searching for any number of thousands of different types of materials 5 with only short paragraphs on each of documented scientific data available in a few volumes.
Previous experience with simulated “real world” samples which are created with controlled known types of explosives is found in such papers as those of Beveridge . But these limited experiments represent a minuscule portion of the controlled experimental data needed.
Quite often the explosives residue analyst hears the expression “Based on my experience” used to satisfy the need for validation of hypotheses. Though relevant, this expression and defense of opinion should never be held to be dispositive by the trier of fact. To begin with, the expert witness who is unwilling to display his data, the real “experience” of the modern scientist, may very well be hiding behind a shallow defense. This defense is not allowed under Daubert. “Ordinarily, a key question to be answered in determining whether a theory or technique is scientific knowledge that will assist the trier of fact will be whether it can be (and has been) tested.” 
How does one test the “experience” of an expert in explosives residue analysis? Without previous hard data to judge from, the trier of fact can be led around by any charlatan who wishes to expound on unsupported conclusions. Counsel faced with this proffer of “evidence,” for instance, in the area of blast effect analysis, might want to ask for data. Counsel should ask just what effects are being measured by the “blast effect” expert when he goes to the crime scene remembering that most large criminal bombings are unprecedented in the effected matrix.
If the expert should choose to talk about the brisance of the explosive and therefore size of material left after the blast, counsel is encouraged to ask what actual measurements were made to determine the size distribution of those materials. When faced with literally tons of rubble, few blast experts have the time to define the rubble size distribution. If the answer counsel receives to questions about blast effect are justified by “my experience,” counsel should keep in mind that the experience of modern science is hard data, not the possibly untested and untestable thoughts of individuals who have not actually taken measurements.
The trier of fact may hear that the explosives residue expert has conducted hundreds, even thousands of analyses on explosives evidence in the past. One must look closely at this type of statement. Again it may have some relevance but should not be dispositive as a validation of hypothesis in a world where every criminal bombing occurs in a different matrix than that before it. 
Evidence from crime scenes does not generally give data from controlled experiments. This type of data simply results in observations from which hypotheses are drawn. It can almost never be used as data to validate future analyses. The valid experience one must use to test hypotheses concerning the identity of the explosive material present in a bomb is the experience gained from initiating many, many explosives in controlled experiments. Counsel should realize that controlled experiments are those in which the experimenter knows beforehand the relevant variables such as what type of explosive was used in the bomb, what kinds of structural materials are in the object destroyed, the placement of the bomb, the prevailing atmospheric conditions, and a myriad of other parameters. The experienced expert who observes blast damage at a crime scene may be able to offer a reasonable hypothesis concerning whether the explosive was a high or low explosive but may be on less solid ground if he offers a more categorical opinion concerning the detonation velocity of the explosive in the bomb.
The type of experience and data needed to render such opinions is extremely expensive and very precious when obtained. The experiments require the making of explosives mixtures, a practice which can be dangerous in itself. The explosive mixture must then be loaded into the type of device that is suspected to have been used in the incident in question. The loading of explosives can be extremely dangerous. And then the whole device must be initiated in a controlled area where the parts of the device recovered will not have residues on them that would not have occurred in the matrix in which the suspect device was initiated. This type of experiment might be conducted on a demolition range used for blowing up explosives. However that range will have residue from other explosions which may contaminate the “controlled” evidence. The whole process is very expensive requiring the coordination of numerous personnel and careful collection of substrates on which residues may exist.
The “expert” who testifies that he has a great deal of experience in explosives residue analysis must be required to describe specifically the experience and how data from that experience validates any hypothesis that he may present to the trier of fact concerning the “identity” of the type of explosive suspected to be present in the device in question. An individual who has detonated many pipe bombs or car bombs should be asked to explain the relevance of that experience and his data to the analysis of data from the explosion of a bomb in an airplane or in a large building such as the World Trade Center. Where a pipe bomb may have the same explosive used in the bombing of a building, the building will still represent an entirely different matrix which most probably will not have been encountered before by the analyst.
But what do the courts have to say about this area of experience, formal education and experimental data acquired? Howse  teaches that the expert “should be received by you with caution. It is largely a field of speculation and requires careful and intelligent evaluation on your part if it is to be valid means of reaching the truth.” Howse also holds that “expert witnesses do not give testimony about a fact which is available for all the world to see and report. Indeed, expert witnesses in most instances do not testify as to a fact on the ultimate issue, but give an opinion based upon the conclusions which they draw from the facts they are able to see or demonstrate. Thus expert witnesses, for the most part, do not testify to “facts” but rather give an opinion based on facts, which facts are quite often obscured from the view of a person not specially trained in the field.”
What have the courts accepted as proper credentials of personnel trained in a discipline and therefore recognized as experts in that discipline? The recent litigation over forensic DNA analysis is instructive here. In Government of the Virgin Islands v. Penn  the court described the credentials of the experts testifying in that matter as follows: “1. Budowle: Bruce Budowle is the FBI’s Program Manager for DNA Research. 2. Robert Coffin has been employed with the FBI for over ten years. Coffin holds a B.S. and M.S. in chemistry with a special emphasis on biochemistry from Ball State University in Muncie, Indiana. After completing his degree programs Coffin worked for three years as a biochemist for an unidentified college. Coffin underwent a six-month training period with respect to DNA profile testing. That training involved college-level courses, performing numerous test and actual cases, all under the supervision of qualified examiners…. Coffin has performed over 130 DNA profile tests. Coffin remains current in the latest journals, articles, and publications relating to DNA profile testing. 3. William M. Shields has been a Professor of Biology with the State University of New York’s College of Environmental Science and Forestry since 1979. He teaches courses in animal behavior, ornithology, conservation biology, conservation genetics, evolution, systematic biology, and population genetics. His research is in the field of animal behavior, behavioral ecology, evolutionary biology with an emphasis on population genetics. He holds a B.A. in biology from Rutgers University, New Jersey and an M.S. and Ph.D. in zoology from Ohio State University. Shields has published numerous papers on population genetics as well as a book on the subject…. Since 1987 Shields has conducted DNA typing of rattlesnakes, Swallow, wolf, deer, and chipmunks.” The court went on to apply the teachings of Daubert to the FBI’s DNA profiling process and conclude the process is both relevant and reliable. At the other end of the spectrum of formal education and experience is Habecker . In this civil matter plaintiff argued on appeal that the district court abused its discretion by refusing to allow the testimony of its expert. The court held that the expert’s testimony was not reliable. The court also noted that the record showed that the expert did not have an engineering degree and had never taken a course in structural engineering or biomechanical engineering. Nor had he had any training in dynamics, physics or kinematics. His experience in mathematics, physics and dynamics was said to have arisen from a layman’s point of view. He testified that he was not a trained mechanic but did have mechanical abilities. And in Berry  the court noted that the problem with the award of a money judgment stemmed from the nature of the “expert” testimony offered by plaintiff citing grave concerns about the qualifications of plaintiff’s witness. The court conducted a detailed review of the witness’ credentials. A reproduction of that review is instructive here. The court noted that the witness received a degree in sociology in 1971 and a master’s degree in education in 1976. He also took courses in criminal justice. His law enforcement career started when he received an appointment as a deputy sheriff in 1966, a position which the witness admitted required no qualifications. The witness did not receive any formal training in this position. He simply started and worked with more experienced officers. That line of work was in general patrol, investigation work, riding a black and white, responding to calls and teaching defensive tactics. The witness did run for election and won but the only necessary qualification for that job was the ability to get elected. The court went on to advised that what the witness did after being a sheriff illustrates the “problem that courts and juries have with expert testimony…. If an expert has a degree in electrical engineering, there are some assumptions that safely can be made relative to his general training…. But when a sociologist cum sheriff is allowed to testify as to all manners of police practices  Berry v. and procedures, the slopes become slippery indeed. The witness’ next job was to work for four years for the Justice Department where he “developed training criteria to train sheriffs and managers of large sheriffs departments. Since up to this point in time he had had little or no training himself, exactly why he got this assignment is difficult to understand…. The fact that some of this would sound impressive to a jury is its vice, not its virtue. The fault lies not with the witness but, rather, with the system. The courts have had a difficult time in appropriately cabining the opinion testimony of ‘scientific’ experts.” In Mobley , the court also disqualified a police officer where the record showed none of the qualifications required to administer a breathalyzer test. The court noted that the officer may have been thoroughly qualified, however the record did not reflect that qualification. The witnesses in Berry and Mobley might not necessarily have been disqualified as experts simply because of their law enforcement background as opposed to their formal education or scientific credentials. They may very well have qualified as experts in fields more closely related to their line of employment. Locascio  involved a challenge to the admission of expert testimony on the inner workings of an organized crime family as being outside the scope of expert testimony. The court found that the FBI agent’s credentials and training were sufficient to qualify him in this particular area. Special Agent Schiliro had been an FBI agent for seventeen years and for five of those years had been on the FBI’s Organized Crime Program, a squad that investigated only organized crime cases. The court noted  that “Rule 702 only requires that an expert witness have “scientific, technical, or other knowledge, skill, experience, training, or education.” The question then arises concerning the degree of expertise required to satisfy the courts that an individual is a qualified expert. Park  teaches us that “the degree and manner of knowledge and experience required of an alleged expert is directly related to the complexity of the subject matter and the corresponding likelihood of error by one insufficiently familiar therewith.” In Grey  the court rejected the defendant’s argument that an employee of the Nebraska State patrol was not an expert on the operation of video poker machines and their use as gambling devices. The expert had testified that he had attended a ten-day training course in Las Vegas, Nevada, on the subject of gambling devices during which course he worked with video poker machines. The expert had also accompanied other agents on several raids during which gambling devices were confiscated. In this simple field of expertise the witness’ training was sufficient for the court to qualify him as an expert. However in more complex fields the courts have taken a more critical stand on the level of training and experience required for an individual to be qualified as an expert. In McLendon  an economic geologist with an expertise in the field of industrial minerals was not qualified to testify about the quantity, quality, or value of kaolin that vendors had alleged was fraudulently concealed by a purchaser. The “expert” advised that he had Bachelors and Masters degrees from the University of Georgia where he took the survey courses in geology, but did not attend a course on clay mineralogy or otherwise study the mineral kaolin in any depth. The court found that his training and education alone did not qualify him to testify as an expert on kaolin. When the “expert” offered that his knowledge, skill, and work experience, in combination with his schooling, qualified him as an expert in kaolin, the court rejected that argument after cross-examination revealed that he had minimal work experience in the particular field of kaolin evaluation despite the fact that he had academic degrees in geology. In Chikovsky  the court even rejected the expert testimony of a medical doctor with a specialty in obstetrics/gynecology. The court noted that the doctor had no specialized training in embryology or teratology, subjects which he was testifying about, did not rely on any published material in forming his opinion, was not a geneticist, and was not aware of any published article or treatise that supported his opinion. The court also noted that the doctor’s scientific theory had not been tested.
The area of “experience” presents one of the most subtle problems associated with explosives residue analysis. The task is made easier by a consideration of the FBI’s guidelines for Evidence Response Teams as well as the integrated approach followed by the Royal Canadian Mounted Police.  These guides describe an integrated team of experts functioning at these highly complex crime scenes. The FBI describes the positions of Team Leader, Photographer, Photographic Log Recorder, Sketch preparer, Evidence Log Recorder, Evidence Custodian, 2 Evidence Collectors/Processors and team members to include specialists (as required). The RCMP describes post blast teams as including a forensic chemist.
Counsel approaching this team for testimony must recognize that the field of explosives residue analysis is extremely complex, requiring in-depth understanding of state-of-the-art technology in chemical analysis and an in-depth knowledge of the significance of data. The understanding of scientists in the field is dramatically changing with new discoveries about instrumental analysis, new kinds of explosives used by terrorists and the techniques required to detect those materials, matrix characterizations, and a myriad of other factors which govern the ultimate scientific opinion. The team is traditionally led by Explosive Ordnance Demolition personnel or personnel who have backgrounds in bombing crime scene investigations. These personnel do not necessarily have technical training but have been crime scene managers. Just as mass disaster scene management would employ excellent managers but not necessarily medical doctors as site managers, so the bombing crime scene management follows the same route. Quite often these individuals have no formal training at all in the physical sciences. The management of the team requires an incorporation of not only residue analysis data but also device reconstruction, photography, evidence collection and storage, materials analysis involving analysis of such materials as tapes, plastics, metals, and paints, as well as law enforcement investigators. The team manager usually does not have an expertise in all or even more than one of these fields. That would be impossible. However that team manager must coordinate operational personnel, provisions of supplies, communications of information from team members to upper-management personnel, travel, and a myriad of management functions not specifically involving science. Counsel that calls such an individual to testify to the significance of technical information will probably experience the frustration of having the “expert” disqualified.
Anderson  notes that the “expert’s background must show qualification sufficient to permit expression of an opinion that is borne of the specialized knowledge or expertise which allows the expert to give opinion evidence…” The FBI Laboratory, for instance, has for the past twenty years, essentially tasked specific individuals full time with only the analysis of explosives residues. Those individuals have had advanced academic credentials in the physical sciences and have utilized state-of-the-art equipment. Simply understanding the equipment much less the science has required extensive and continual training by those individuals. Calling anyone to the stand to testify concerning explosives residues except qualified experts would be inviting objection and a basis for appeal unless that individual’s credentials were thoroughly established. Counsel must be alert to the true meaning of “explosives expert.” An individual could have been in combat and seen thousands of bombs blow up at close range and witnessed the destructive capability of explosives and have an expertise in explosives. However that expertise would not qualify the individual to testify about the analysis of explosives residues.
The point above is illustrated well in Berry.  The court states, “The distinction between scientific and non-scientific expert testimony is a critical one. By way of illustration, if one wanted to explain to a jury how a bumblebee is able to fly, an aeronautical engineer might be a helpful witness. Since flight principles have some universality, the expert could apply general principles to the case of the bumblebee. Conceivably, even if he had never seen a bumblebee, he still would be qualified to testify, as long as he was familiar with its component parts. On the other hand, if one wanted to prove that bumblebees always take off into the wind, a beekeeper with no scientific training at all would be an acceptable expert witness if a proper foundation were laid for his conclusions. The foundation would not relate to his formal training, but to his firsthand observations. In other words, the beekeeper does not know any more about flight principles than the jurors, but he has seen a lot more bumblebees than they have.”
Now look at the task of conducting a bombing crime scene investigation. The team can be composed of a number of different individuals including specialists. The area of expertise of those specialists must be identified and counsel must understand the required credentials and training needed to fill those positions. The FBI Laboratory has represented for many years an example to follow in the forensic community.
Just as the FBI Laboratory has taken the lead in forensic DNA analysis development and presentation so has it taken the lead in other areas of forensic analysis. The level of training that this Laboratory has required of its experts to practice in a particular field of forensic science has successfully stood the test of close judicial scrutiny for over sixty years. For example, one may look at the credentials of the DNA examiners described in Penn  above, noting the amount of in-house training of the FBI DNA experts despite their own impressive credentials in formal education.
The FBI bombing crime scene investigator who discovers paint which may have probative value gives that paint to an examiner who has spent nine months to a year in training in the analysis of paint. Adhesive tape evidence from these crime scenes is entrusted to individuals with extensive training in tape and tape analysis. Each minor area of investigation requires extensive and continual training. The area of explosives residue analysis is no different and in fact is much more extensive. The author, despite holding a Ph.D. in chemistry, spent fourteen months in training in explosives analysis before being qualified by the FBI Laboratory to examine such evidence. The author’s predecessor, also a Ph.D. chemist with the FBI Laboratory, spent a year in training in explosives analysis. And the author’s successor, who holds a Masters in chemistry, also spent a year in training before being qualified as an explosives analysis expert.
The FBI obviously has taken this training and level of expertise very seriously as should counsel in preparation for court. Counsel should recognize that individuals trained in the forensic analysis of drugs or arson or paint, for instance, may be chemists but do not necessarily have the requisite knowledge to render valid opinions about such fields as polymers, lubricants, DNA analyses, or explosives residue analysis. In order for the expert in explosives residue analysis to arrive at a valid scientific conclusion utilizing the explosives residue analysis protocol presently employed at the FBI Laboratory she must have a broad scientific knowledge base. She must have a working knowledge of optical microscopy, scanning electron microscopy, energy dispersive x-ray analysis, x-ray fluorescence spectrometry, inductively coupled plasma spectrometry, emission spectrometry, capillary electrophoresis, high performance liquid chromatography, gas chromatography, chemiluminescence detection, electron capture detection, flame ionization detection, Fourier transform infrared spectrometry, mass spectrometry with introduction techniques of gas chromatography, solids probe, and pyrolysis, nuclear magnetic resonance spectrometry, ion chromatography, and atomic absorption spectrometry. She must be current on a working knowledge of state of explosives detection equipment.  She must maintain a current working knowledge of these fields. She must understand the possible matrices that explosives may interact with. She must realize the enormous complexity of accidental contamination issues and environmental contamination issues. She must know the types of possible materials that exist in different environments before a bomb explodes that might lead her to misinterpret analytical results. She must understand the manner in which various chemicals react with their environment both in the world and in the chemical analysis instruments. She must also understand very, very clearly what she does not analyze for or know in the area of explosives residue analysis.
An example from the civil case law is instructive. Datskow  involved a products liability action against an airplane engine manufacturer arising from a fatal airplane crash. An investigation of the scene of the accident and subsequent testimony concerning the cause of the crash required the testimony of Patrick McGinley, an expert in fire investigation. The court tells us that “He concluded from his examination of the airplane wreckage that a fire began in the engine while the plane was still in flight….” In criminal bombings of aircraft one often sees what appears to the untrained eye of the layman, to be fire damage. But when the court needs expert testimony to discern the significance of this “fire damage” the court would not qualify an explosives residue expert, or a paint expert, or a DNA expert, or an adhesives expert to render an opinion concerning the significance of the burn patterns. The court would qualify an expert in fire investigation to render an opinion concerning the burn patterns in the wreckage. Markum  also teaches that “The very fact that fire departments have specialized arson investigation units argues that arson is an area of technical and specialized knowledge beyond the ken of the average juror.” And if questions concerning the death of the victims or the state of their bodies arose in such an incident, the court would not qualify an explosives expert or a paint expert, or a polymer expert, to testify to matters involving forensic pathology. As the court noted in Datskow , the defendant “offered witnesses who offered opinions on fire, weather conditions, piloting an aircraft, the cause of death and so on.”
Courts have traditionally recognized separate areas of expertise and been able to recognize when witnesses offered as “experts” were not in reality what they offered themselves to be. However in the area of explosives expertise there may be misunderstandings on the court and counsel’s part concerning the significance of the phrase “explosives expert.” Counsel should very closely give this area of expertise a “hard look” . Counsel that may fall into the trap of merely accepting credentials due to the seemingly difficult task of trying to discern just what is an “expert” should learn from Glaser . Glaser notes that the “one expert for the plaintiff who testified that the drug did cause the injury had based his testimony on ‘personal belief or opinion’, not ‘on the basis of the collective view of his scientific discipline’, nor by explaining coherently the ‘grounds for his difference’ with his scientific peers.” Determining this fact required substantial effort on the part of a court which is proficient in the law while probably not in areas of science.
Though Janopoulos  teaches that experts are allowed to “rely in part on information supplied by another expert” this does not relieve counsel of determining if the “expert” offered to the court is indeed an expert in the field in which he will testify. A medical doctor who had treated wounded personnel at a bombing crime scene would not be offered as an expert to testify to the significance of damages to the structural integrity of a bombed building.
When counsel recognizes the power of the expert witness over the jury as did the court in Collins  counsel must also realize that close scrutiny of the foundation for expert witness testimony involving explosives is mandatory.
 Executive Director, Forensic Justice Project, Washington, D.C., B.S. Chemistry, 1974, East Carolina University, Ph.D. in Chemistry, 1980, Duke University, J.D., 1996, Georgetown University School of Law. (202)342-6980.
 The Picatinny Arsenal, Encyclopedia Of Explosives And Related Items, supra note 72, should be referred to by the serious student of explosives residues in order to see the number of types of explosives available to the criminal bomber.
 Beveridge, supra note 10.
 Daubert, supra note 17 at 2796.
 The author has conducted chemical analyses at the FBI Laboratory since 1986 on thousands of pieces of explosives evidence in over nine hundred explosives related matters and never seen in any of these cases matrix duplication.
 State v. Howse, 634 S.W.2d 652 (1982).
 Government of the Virgin Islands v. Penn, 838 F.Supp.1054 (1993).
 Habecker v. Clark Equipment Co., 36 F.3d 278 289(3rd Cir.1994).
 City of Detroit, 25 F.3d 1342 (6th Cir. 1994).
 State v. Mobley, 160 S.E.2d 334 (1968).
 U.S. v. Locasio, 6 F.3d 924 (2nd Cir. 1993).
 Id. at 937.
 People v. Park, 380 N.E.2d 795 798 (1978).
 U.S. v. Grey, 856 F.Supp. 1515, 1521 (1994).
 McLendon v. Georgia Kaolin Co., Inc., 841 F.Supp. 415(M.D.Ga. 1994).
 Chikovsky v. Ortho Pharmaceutical Corp., 832 F.Supp. 341(S.D.Fla. 1993).
 J.S. Deak, H. Clark, C. Dagens, J.J. Gaudet, B.W. Richardson, Processing the Post-Blast Scene; An Integrated Approach, Proceedings Third Symposium On Analysis And Detection Of Explosives, July 10-13, 1989, supra note 14.
 Anderson, supra note 76, at 943.
 Berry, supra note 122, at 1349.
 Penn, supra note 88. 89
 In United States v. Lee, 25 F.3d 997 (11th Cir. 1994) the defense contended that the court improperly admitted evidence from two instruments used to detect trace amounts of cocaine on Lee’s personal effects. Both of these instrumental techniques have been used in the detection and analysis of explosives. One of the instruments, the Ionscan, which is based upon ion mobility spectrometry, has been found to give significant numbers of false negatives and positives when used with forensic, “real world,” explosives residue samples. The technology used by the otheri nstrument, the Sentor, which is gas chromatography chemiluminescence, has in the manner used by the explosive detection, also been found to give significant numbers of false positives when used on forensic samples. Without a very clear understanding of the failure rate of these instruments, the untrained, unqualified “expert” could very well mislead the trier of fact to believe that explosives and/or drugs were actually on a substance.
 Datskow v. Teledyne Continental Motors, 826 F.Supp. 677(W.D.N.Y. 1993).
 U.S. v. Markum, 4 F.3d 891 895 (10th Cir. 1993).
 Datskow, supra note 135 at 683.
 Turpin v. Merrell Dow Pharmaceuticals, Inc., 1349 F.2d1349 1352 (6th Cir. 1992).
 Glaser v. Thompson Medical Co., Inc., 32 F.3d 969 975(6th Cir. 1994).
Janopoulos v. Harvey L. Walner & Associates, 866 F.Supp.1086 1095 (N.D.Ill. 1994).
 People v. Collins, 438 P.2d 33 40(1968). In Collins the court stated that “Confronted with an equation which purports to yield a numerical index of probably guilt, few juries could resist the temptation to accord disproportionate weight to that index; only an exceptional juror, and indeed only a defense attorney schooled in mathematics, could successfully keep in mind the fact that the probability computed by the prosecution can represent, at best, the likelihood that a random couple would share the characteristics testified to by the People’s witnesses-not necessarily the characteristics of the actually guilty couple.”