the Nature of Evidence
Evidence is any statement or material object from which reasonable conclusions can be drawn. It is a broad category embracing anything perceptible to the five senses including documents, exhibits, facts agreed to by both sides, and the testimony of witnesses. Evidence in a criminal trial concerns the intent, motive, means, and opportunity to commit a crime.
In general, evidence is divided into two categories： circumstantial and physical. Circumstantial evidence consists of information gleaned from witnesses and documents that point to an individual as the perpetrator of a crime. Physical evidence consists of actual objects ？bodies, weapons, body fluid stains, fingerprints, hairs, fibers, etc. ？that are associated with the crime and may be linked to the perpetrator.
It is the work of forensic scientists to examine the physical evidence, and using the methods of science, to reconstruct the events that constituted the crime. The prosecutor must then combine this data with statements of witnesses and evidence from documents such as correspondence, telephone records and credit card receipts to develop an overall theory of the case which can be presented in court.
Scientific evidence is an increasingly important part of both civil and criminal trials. Forensic science is a growth industry. New technologies for analyzing physical evidence are growing rapidly and private companies are becoming an increasingly important resource for the legal system. The testimony of experts is the primary means of introducing scientific evidence. Because these experts are imparting information "beyond the ken" of the layperson, they must present information that goes beyond first hand observation, opinions and hearsay not permitted under ordinary rules of evidence. Lay witnesses are constrained to testify only about matters they have directly observed. Expert witnesses are allowed to draw inferences from facts which the judge or jury is not competent to draw. They may also rely on seminars, publications, records and conversations with other experts that are part of their normal course of business.
Discovery
Despite fictional presentations to the contrary （Perry Mason is a prime example）, there are very few surprises in actual trials. This is because of the process called discovery, whereby opposing attorneys are permitted to learn the facts and expert opinions upon which the other side is basing its case prior to the actual trial. In addition, each side is required to provide the other side with a list of its witnesses before the start of trial.
Providing discovery materials in criminal cases is binding only upon the prosecution in all but a few states and Canada. Access to materials through the discovery process is the main avenue the defense has for learning what evidence will be presented against the accused at trial. This allows the defense to re-examine the evidence and develop alternative hypotheses to the prosecutor's case.
In California, the defense's access to scientific evidence is defined in the Griffin decision which provides that the defense can have the evidence only after the prosecution has completed their testing. Also, under both Griffin and a U.S. Supreme Court decision in Arizona v. Youngblood, the prosecution may consume the evidence in the testing process, as long as they act in good faith.
Beginning in 1989, furious battles erupted over discovery efforts in the DNA war. Gen-erally the defense has been able to examine autoradiographs from the case in question, laboratory reports, and the lab notes that support them in addition to the lab procedure manuals and proficiency testing results. Requests for additional materials such as other autoradiographs, validation studies, population data bases, and raw data face harsher scrutiny and often were not honored. Early DNA cases were marked by long and costly litigation over discovery. The defense claims that the prosecution and the labs they employ "stonewall' discovery requests. The lab resist discovery, maintaining that the requested materials are privileged, constitute trade secrets, are legally irrelevant.
Forensic labs also claim that the defense regularly makes overly burdensome and duplicitous requests for reams of material. If they were required to comply, lab personnel would be doing little else than identifying and duplicating discovery materials. DNA discovery battles are still being strenuously fought. Indeed, O.J. Simpson's chief lawyer, Robert Shapiro, has labeled Cellmark a "discovery outlaw." However most commentators would agree that many of the issues surrounding discovery already have been litigated or settled in other ways.
Scientific Evidence Admissibility Standards
The key element in whether scientific evidence is admissible is whether it is trustworthy. To be considered trustworthy, it must demonstrate accuracy （validity） and consistency （reliability）. Admissibility is determined by the Frye rule, which stresses "general acceptance" or by the Federal Rules of Evidence （followed by some state courts） which stress helpfulness, reliability, and relevance.
In all of the trials to date in which DNA evidence has been involved, courts have ruled it as evidence or, on appeal, have remanded the case to the trial court in 22 reported cases and have limited its admissibility in 16 cases, generally because of statistical questions.
The Frye Standard
In the 1923 decision United States v. Frye, a District of Columbia circuit court ruled against the admissibility of lie detector evidence in a murder case because the technology had not been accepted in the relevant scientific community. Since then, most state courts have followed this general standard on whether or not to allow novel scientific evidence. The so-called Frye hearing gives the prosecution and defense the opportunity to attack adverse scientific evidence and try to keep it out of the trial. The key paragraph in this decision reads：
Just when a scientific principle or discovery crosses the line between experimental and demonstratable stages is difficult to define. Somewhere in the twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs. Determining "general acceptance" according to the Frye standard is a two-step procedure： （1） identifying the particular field（s） into which the scientific principle or discovery falls and the relevant scientific community； and （2） determining whether that community accepts the technology, principle, or discovery. Both the underlying theory and the procedures used to produce results must be generally accepted by scientists in the relevant fields.
To these two criteria has been added a third in some jurisdictions. In California, the additional standard evolved from the 1976 decision in People v. Kelly which held that "the proponent of the evidence must demonstrate that correct scientific procedures were done in the particular case." This third "prong" also was accepted by the court in the 1989 landmark New York v. Castro, the first case in history of where DNA evidence was excluded. A distinction lost on some courts is that the Kelly rule only requires that correct procedures be used, not that the court must determine that these procedures were performed correctly.
Legal evidence rules generally hold that how well work is performed should not be the subject of an admissibility hearing because the quality of testing in a particular case goes to the weight of the evidence and not its admissibility. It is up to the trier of fact, the judge or the jury, to determine how much weight or consideration to give that evidence. In practice, the legal distinction between the admissibility of scientific testing and the weight that
should be given that testing has become increasingly blurred in DNA evidenciary hearings.
Evidence is any statement or material object from which reasonable conclusions can be drawn. It is a broad category embracing anything perceptible to the five senses including documents, exhibits, facts agreed to by both sides, and the testimony of witnesses. Evidence in a criminal trial concerns the intent, motive, means, and opportunity to commit a crime.
In general, evidence is divided into two categories： circumstantial and physical. Circumstantial evidence consists of information gleaned from witnesses and documents that point to an individual as the perpetrator of a crime. Physical evidence consists of actual objects ？bodies, weapons, body fluid stains, fingerprints, hairs, fibers, etc. ？that are associated with the crime and may be linked to the perpetrator.
It is the work of forensic scientists to examine the physical evidence, and using the methods of science, to reconstruct the events that constituted the crime. The prosecutor must then combine this data with statements of witnesses and evidence from documents such as correspondence, telephone records and credit card receipts to develop an overall theory of the case which can be presented in court.
Scientific evidence is an increasingly important part of both civil and criminal trials. Forensic science is a growth industry. New technologies for analyzing physical evidence are growing rapidly and private companies are becoming an increasingly important resource for the legal system. The testimony of experts is the primary means of introducing scientific evidence. Because these experts are imparting information "beyond the ken" of the layperson, they must present information that goes beyond first hand observation, opinions and hearsay not permitted under ordinary rules of evidence. Lay witnesses are constrained to testify only about matters they have directly observed. Expert witnesses are allowed to draw inferences from facts which the judge or jury is not competent to draw. They may also rely on seminars, publications, records and conversations with other experts that are part of their normal course of business.
Discovery
Despite fictional presentations to the contrary （Perry Mason is a prime example）, there are very few surprises in actual trials. This is because of the process called discovery, whereby opposing attorneys are permitted to learn the facts and expert opinions upon which the other side is basing its case prior to the actual trial. In addition, each side is required to provide the other side with a list of its witnesses before the start of trial.
Providing discovery materials in criminal cases is binding only upon the prosecution in all but a few states and Canada. Access to materials through the discovery process is the main avenue the defense has for learning what evidence will be presented against the accused at trial. This allows the defense to re-examine the evidence and develop alternative hypotheses to the prosecutor's case.
In California, the defense's access to scientific evidence is defined in the Griffin decision which provides that the defense can have the evidence only after the prosecution has completed their testing. Also, under both Griffin and a U.S. Supreme Court decision in Arizona v. Youngblood, the prosecution may consume the evidence in the testing process, as long as they act in good faith.
Beginning in 1989, furious battles erupted over discovery efforts in the DNA war. Gen-erally the defense has been able to examine autoradiographs from the case in question, laboratory reports, and the lab notes that support them in addition to the lab procedure manuals and proficiency testing results. Requests for additional materials such as other autoradiographs, validation studies, population data bases, and raw data face harsher scrutiny and often were not honored. Early DNA cases were marked by long and costly litigation over discovery. The defense claims that the prosecution and the labs they employ "stonewall' discovery requests. The lab resist discovery, maintaining that the requested materials are privileged, constitute trade secrets, are legally irrelevant.
Forensic labs also claim that the defense regularly makes overly burdensome and duplicitous requests for reams of material. If they were required to comply, lab personnel would be doing little else than identifying and duplicating discovery materials. DNA discovery battles are still being strenuously fought. Indeed, O.J. Simpson's chief lawyer, Robert Shapiro, has labeled Cellmark a "discovery outlaw." However most commentators would agree that many of the issues surrounding discovery already have been litigated or settled in other ways.
Scientific Evidence Admissibility Standards
The key element in whether scientific evidence is admissible is whether it is trustworthy. To be considered trustworthy, it must demonstrate accuracy （validity） and consistency （reliability）. Admissibility is determined by the Frye rule, which stresses "general acceptance" or by the Federal Rules of Evidence （followed by some state courts） which stress helpfulness, reliability, and relevance.
In all of the trials to date in which DNA evidence has been involved, courts have ruled it as evidence or, on appeal, have remanded the case to the trial court in 22 reported cases and have limited its admissibility in 16 cases, generally because of statistical questions.
The Frye Standard
In the 1923 decision United States v. Frye, a District of Columbia circuit court ruled against the admissibility of lie detector evidence in a murder case because the technology had not been accepted in the relevant scientific community. Since then, most state courts have followed this general standard on whether or not to allow novel scientific evidence. The so-called Frye hearing gives the prosecution and defense the opportunity to attack adverse scientific evidence and try to keep it out of the trial. The key paragraph in this decision reads：
Just when a scientific principle or discovery crosses the line between experimental and demonstratable stages is difficult to define. Somewhere in the twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs. Determining "general acceptance" according to the Frye standard is a two-step procedure： （1） identifying the particular field（s） into which the scientific principle or discovery falls and the relevant scientific community； and （2） determining whether that community accepts the technology, principle, or discovery. Both the underlying theory and the procedures used to produce results must be generally accepted by scientists in the relevant fields.
To these two criteria has been added a third in some jurisdictions. In California, the additional standard evolved from the 1976 decision in People v. Kelly which held that "the proponent of the evidence must demonstrate that correct scientific procedures were done in the particular case." This third "prong" also was accepted by the court in the 1989 landmark New York v. Castro, the first case in history of where DNA evidence was excluded. A distinction lost on some courts is that the Kelly rule only requires that correct procedures be used, not that the court must determine that these procedures were performed correctly.
Legal evidence rules generally hold that how well work is performed should not be the subject of an admissibility hearing because the quality of testing in a particular case goes to the weight of the evidence and not its admissibility. It is up to the trier of fact, the judge or the jury, to determine how much weight or consideration to give that evidence. In practice, the legal distinction between the admissibility of scientific testing and the weight that
should be given that testing has become increasingly blurred in DNA evidenciary hearings.