The effective expert evidence

Smt Sharada Avadhanam ADFS APPA.

Judges/ judiciary world over are now basing their verdict by placing a lot of credibility on circumstantial evidence as submitted by the expert, to overcome the inherent problems of perjury and hostile witnesses.

Circumstantial evidence relates to other facts from which the facts in issue of the case may be inferred, and establishes the commission of offence by proving circumstances which lead to credible and irrefutable inference of guilt.

Expert evidence is the evidence, the relevance and probative value of which, depends upon the witness having some special knowledge or skill other than having directly perceived a fact in a case.

Forensic scientist must try to assess the value of evidence consisting of simple non-replicable items of information about specific hypothesis referring to an individual event.

The standard of proof required to convict a person on circumstantial evidence is established by a series of decisions of Supreme Court of India, viz:

- circumstances must be fully established

- Chain of evidence must be complete.

- It must be such as to show that within all human probability the act must have been done by the accused.

- When deciding the question of sufficiency the court considers the total cumulative effect of all the proven facts.

- Experts must be competent to depose and must have special skills therein. The competency is often decided by the presiding judge.

The abstract of analysis can be construed as opinion in case they reflect the personal opinion of the expert through the act of general skills, experience duly substantiated, whereas the report of scientific analysis is the one generated out of time-tested, and peer-reviewed experiments of general acceptance.

In viewing the expert evidence a word of caution is exercised by the judges. Its value is only corroborative but not substantive evidence.

In establishing the standards for such evidence several criteria were laid down by Supreme Court from time to time. Daubert’s criteria and Fry’s are the classical ones in the series.

The expert must not give evidence on legal concepts, as often the courts insist that an expert should not give evidence which involves interpreting and applying a legal concept.

- Eg: the medical witness could not therefore state whether the accused was suffering from a disease of the mind. The role of the witness was to make a medical diagnosis and explain to the court the ramifications of that diagnosis. The judge would then decide whether the condition could amount to a disease of the mind.

A scientific witness should only express the strength of a particular piece of evidence and not give an opinion on the issue. In the words of Hodgkinson, expert evidence must have high probative value in order to be admissible. In some cases ultimate issue is a matter of legal categorization and not of scientific expertise. An example of this is definition of insanity.

Scientific evidence is always to be combined with other evidence relevant to the same issues and does not stand alone.

The major problem raised by the scientific evidence is how it is to be interpreted. If experiments and testing procedures were only done correctly, the scientist would produce the right answer.

It is common for the defense to spend considerable energy to a detailed examination of the testing process in the hope of uncovering some failure of procedure, which might cast doubt on the accuracy of the evidence. A good quality control regime combined with good laboratory procedures, which are integral part of any ISO and NABL certified lab, is the remedy to counter such allegation by the defense.

The other weapon in the armoury of defense is to point the standards of care exercised in handling and analyzing the samples. A sample found at the crime scene may already be contaminated or degraded. it should be handled in such a way that no further contamination or degradation occurs and it is also equally important to be able to prove that the sample analyzed is indeed the same taken from the crime scene. Continuity is assured by good handling, lifting procedures by adopting proper chain of custody, keeping the integrity of the sample / material intact.

Well documented procedures, as a prerequisite of ISO certification are in place, through which certified labs strive to improve their quality standards e.g.: refusing to analyze samples which are not properly packed and apparently tampered with.

Internal audits and blind and open proficiency tests are again checks on the quality of testing procedures.

Therefore the

- evidence must be considered in the light of the prior odds and other evidence

- Evidence must not be double counted, that is, it should not be used more than once in relation to any one issue, but can be used for more than one purpose.

Ultimately the analysis is no better than the actual sample analyzed. If there are lapses on the part of the IO either in sending the sample, non collection of control or wrong collection of sample, not following a particular legal procedure in hading over or receiving an exhibit, or tempering with the evidence item, which adversely hamper the evidentiary value of the item in hand eventually making it inadmissible in the courts of law.

In case of valid alternate hypotheses emerging from the scientific evidence on hand, the expert should not reject them out of hand but weigh the probabilities in favour of each hypothesis and indicate his considered judgement. He should clearly state the reasons for rejection of alternate scenarios as also the reasons for his chosen scenario. The defense which works on alternative hypotheses and alternative scenarios to the prosecution case can only be countered in the above fashion and not by a-priori assumption, which do not stand the scrutiny of the court.

The general view is that there is a failure of communication between the scientific witness and the lawyers. The lawyers contend that scientists do not understand the legal process while the scientists claim that they are prevented from giving the evidence that ought to be given.

In conclusion

- correct interpretation by the court: understanding the logic of the inference is more important to lawyers than technical knowledge about scientific processes

- inference is a matter of logic and can and should be understood by everybody involved with the legal process

- scientific evidence relating to an issue should be combined with other evidence relating to the same issue

- Evidence has a value only in a context. its value depends on it’s ability to distinguish between specific hypotheses

The best way to express the weight of an item of evidence is on the basis of the numerical values, the likelihood ratio.