Forensic science is a scientific discipline that functions within the parameters of the legal system. Its purpose is to provide guidance to those conducting criminal investigations and to supply to courts accurate information upon which they can rely in resolving criminal and civil disputes. To this extent it exists for, and also is regulated by, the law, both statutory and decisional.
Scientific analysis for forensic purposes can only lawfully take place on samples that have been properly procured. Forensic science evidence may be determined inadmissible on the basis of its provenance and, in particular, on the ground that it has been illegally or improperly obtained. If samples from the body of a suspect and a victim, be they intimate or nonintimate body samples, have been obtained by, for instance, an unlawful search or a false imprisonment of a suspect by law enforcement officers, evidence relating to the results of their scientific analysis may not be admissible. However good the scientific work, the law may prohibit the admission of the evidence by reason of the impropriety.
Scientific evidence intended to be laid against an accused person by the prosecution in criminal litigation must generally be disclosed to the accused or his or her legal representatives before trial. This is a basic requirement of fair process, so that the accused is in a position to counter it or to have a second test undertaken on the samples in order to evaluate the strength of the prosecution’s case. However, scientific techniques do not always permit such a process. Sometimes, of their nature, they destroy or damage the sample available for testing, thereby precluding or impairing the potential for replication of analysis. This also has ramifications for the admissibility of the results of the testing.
Similarly, gaps in the chain of custody of exhibits later scientifically examined may lead to the inadmis-sibility of evidence of the results of the scientific tests because they so significantly impair the probative value of the evidence.
A number of variable factors bear upon the admissibility of the results of, and the utility of the fruits of, forensic science testing. Included among these are matters such as whether the scientist involved is sufficiently expert in the area; whether the area is one of expertise, be that determined by what is generally regarded within the area as accepted or by reference to the reliability of the theory or technique; whether the trier of fact needs or would benefit from the evidence; whether the evidence bears upon an ultimate question to be decided by the trier of fact; and whether the bases of the expert’s opinion have been properly proved. For those jurisdictions where such matters are not threshold matters for admissi-bility, each functions as a measure by which the probative value of the forensic science evidence can be evaluated.
In many jurisdictions forensic science evidence may also be adduced without the scientist involved actually giving oral evidence, unless the person is required to be available for cross-examination by the side to whose detriment the evidence is given. This is a means of shortening proceedings and facilitating the provision of evidence where the processes of cross-examination do not need to be invoked.

Search and Seizure

Rules on evidentiary admissibility vary from jurisdiction to jurisdiction and legal system to legal system. However, it is common for courts to decline to receive evidence presented in a criminal case on behalf of the prosecution if it has been improperly obtained by law enforcement officers. By statute, and in some jurisdictions by common law, police are entitled to search the persons and the property in the possession of suspected persons. However, there are criteria for obtaining the right to engage in such intrusive exercises, be they obtaining blood or buccal samples for DNA analysis, taking pubic hair samples for matching with crime scene samples, seizing banknotes in the possession of a suspect or examining tattoos on the body of the suspect. Sometimes these involve obtaining warrants from a judicial officer on the basis of information placed on oath before the officer that there is good cause for granting such a warrant.
Where the legal criteria are breached, the fruits of the search and seizure can be described as those of the poisoned tree of an improper search and seizure. The result can be a determination that the direct fruits, namely what has been taken during the improper search and seizure, are inadmissible as evidence against the accused person. If this is so, the same result will generally follow for the indirect fruits, namely the results of scientific tests done on the improperly seized samples.

Chain-of-Custody Rules

It is essential from a legal point of view for continuity of custody of samples to be demonstrated by the prosecution from the time of their being obtained to the time that the samples are tested. It needs to be shown that there is an unbroken chain of custody of the physical sample from the time of seizure to the time of its analysis. If there is any gap in the chain, it means that the sample that was examined and tested may not be the sample seized at the claimed time from the potential scene of crime or association with the accused. The consequence is that another sample may have been tested or that there may have been tampering or contamination of the sample, resulting in inaccuracy or at least a finding unfairly prejudicial to the interests of the accused.
Ultimately, the question is one of relevance. If it cannot be proved that the sample tested was the sample seized, then the evidence is irrelevant: it does not rationally affect the assessment of the existence of a fact in issue in the proceedings. Approaching the issue in another way, if the continuity of evidence is not sufficient to establish an unbroken chain of custody, scientific evidence about the results of testing the sample is likely to be highly prejudicial against the accused person but lacking in substantial probative value. This too permits a ruling that the evidence is inadmissible.

Disclosure of Expert Reports

Rules relating to the extent of disclosure of scientific testing conducted by scientists for the prosecution and for the defense vary. Generally, it is incumbent upon the prosecution to disclose the results of testing, whether or not it favors the case for the prosecution. This prevents the defense from being taken by surprise during a trial and also puts the defense on notice of anomalies of scientific testing that may cast doubt on the evidence being laid on behalf of the prosecution.
Historically, accused persons were under no obligation under Anglo-American law to disclose any aspect of their case, including scientific evidence which they intended to adduce or upon which they intended to rely. However, in some jurisdictions, such as in some states in Australia, this is changing. Accused persons are obliged to disclose the substance of scientific evidence upon which they intend to rely at trial. Failure to make disclosure can result in judicial rulings that accused persons cannot adduce the undisclosed scientific evidence, or in costs orders against the accused because of the adjournments necessitated by the late notice to the prosecution. The change to the traditional right on the part of the accused not to have to make any form of answer to the prosecution case has been justified on the basis of the unfairness of the defense taking the prosecution by surprise and the socially unacceptable consequences of the prosecution being unable effectively to meet scientific evidence adduced on behalf of the accused.

Destruction by Testing of a Sample

Poor laboratory work or the utilization of certain tests may result in the destruction of a sample. When this occurs, it can preclude second testing of the sample by the defense, with the potential for unfairness. If a test unnecessarily, or deliberately, destroys all of a sample, or if records are not kept which permit transparency of the testing process, this too can result in a ruling in some jurisdictions that the results of the scientific testing should be regarded as inadmissible. This is most likely where the destruction of the sample or the failure to retain records was deliberate or the result of egregious carelessness.

Evidentiary Admissibility Rules

Scientific evidence is expert evidence in the form of fact and in the form of opinion. That which is in the form of fact is admissible on the same basis as other lay opinion evidence. That which is opinion is admissible subject to evidentiary rules that differ from jurisdiction to jurisdiction. Most courts, however, require that a scientist giving expert opinion evidence be shown to be an expert by reason of specialist knowledge arising from sufficient experience, skill or training. Rigorous application of this rule can have repercussions for scientists of limited experience or whose primary expertise lies in an area of work other than that in respect of which their expertise is being asserted in a particular case. It is only when entitlement to express opinions as an expert is proved to the satisfaction of a trial judge that such opinions are admissible.
In a number of jurisdictions, reliability of theories and techniques is a condition precedent to the admis-sibility of scientific evidence. This is an approach that has been repudiated in England. In some jurisdictions, though, such as Canada, the US Federal Courts and some states in the United States, check-lists for ascertaining reliability have been developed, including factors such as: the potential rate of error; the existence and maintenance of standards; the care with which the scientific technique has been employed and whether it is susceptible to abuse; whether there are analogous relationships with other types of scientific techniques that are routinely admitted into evidence; the presence of failsafe characteristics; the expert’s qualifications and stature; the existence of specialized literature; the novelty of the technique and its relationship to more established areas of scientific analysis; whether the technique has been generally accepted by experts in the field; the nature and breadth of the inference adduced; the clarity with which the technique may be explained; the extent to which basic data may be verified by the court and jury; the availability of other experts to evaluate the technique; and the probative significance of the evidence.
In Australia, an area of expertise test replicating the ‘general acceptance test’ within the scientific community exists in a number of states, meaning that the inquiry of the courts is as to whether the theories and techniques spawning test results are generally accepted within the relevant scientific community. In the United States the general acceptance test of Frye v. US continues to apply in many jurisdictions, while at federal level the Supreme Court decision of Daubert v. Merrell Dow Pharmaceuticals Inc. 125 L Ed (2d) 469; 113 S Ct 2786 (1993) has prescribed four indicia of reliability: (1) whether scientific evidence can be or has been tested, namely its falsifia-bility, refutability or testability; (2) whether the theory or technique has been subjected to peer review and publication as a means of increasing the likelihood that substantive flaws in methodology will be detected; (3) the known or potential rate of error and the existence and maintenance of standards controlling the technique’s operation; and (4) whether a technique has gained general acceptance within the scientific community. The test therefore evaluates reliability of opinion evidence by a combination of scientific analysis and deference to the views of its legitimacy within the relevant intellectual marketplace. It has been accepted by decisions of the High Court in New Zealand.
In a number of jurisdictions, scientific evidence is not admissible if it is a matter of common knowledge, which has been variously interpreted to mean that it is inadmissible if it either does not assist the trier of fact or if it is not needed by the trier of fact. This is a function of the waxing and waning levels of understanding of scientific, statistical and other concepts by members of the general community.
In some jurisdictions, scientific opinion evidence based upon data which are not proved to the court is inadmissible. In others it is simply accounted as of minimal probative value. The reason for such an approach is that if scientists rely significantly upon others’ opinions or work, the trier of fact is ill positioned to evaluate the strength of their opinions; their views are derivative and not susceptible of informed analysis by a judge or jury.
In some, albeit a reducing number of, jurisdictions, scientists and other expert witnesses are precluded from offering views on the ultimate issues to be determined by the trier of fact. This is a lingering manifestation of a concern to stop expert witnesses from ‘usurping’ the role of the trier of fact. It springs from a pessimistic view of jurors’ capacities for reasoned evaluation of expert evidence, not overborne by the impressiveness and articulateness of expert witnesses. The rule is seldom applied in those jurisdictions where it still exists and is regarded by many commentators as ill-conceived and unhelpful.
In many jurisdictions, scientific opinion evidence can be excluded by the trial judge if admitting it would result in evidence more prejudicial than it is probative going before the trier of fact. Criteria for applying this exclusionary mechanism differ from jurisdiction to jurisdiction, and from case to case, but have the potential to include factors relevant to the application of each of the exclusionary rules previously described. If the probative value of scientific evidence is low but its prejudicial value high, it may well be excluded as evidence. The situation becomes more complex as its probative value rises, and its prejudicial value decreases. In some jurisdictions, such as Australia and England, the tendency is for most evidence, save that which is highly prejudicial, to be admitted and the assessment of its weight is left for the trier of fact to assess.
Each of the exclusionary mechanisms is applied with greatest rigor where it is a lay jury that is the trier of fact. In those jurisdictions where a professional magistrate or judge is the trier of fact in criminal offences, without a jury, the expert evidence rules of exclusion tend to be less stringently applied.

Certification Procedures

In a number of jurisdictions, the law allows for scientific evidence to be tendered by the prosecution upon production of a report to the court and the accused person a suitable time before the trial. Often proof of the author’s credentials to be accounted an expert is required. If the accused wishes to challenge the opinions expressed by the scientist, or the scientist’s observations or the methodology of his or her testing, the onus is upon the accused to call for the presence of the scientist for cross-examination. Such a procedure is designed to streamline evidence that is frequently noncontentious and to save time and money. It is a procedure likely to be utilized more widely.

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