Challenging forensic evidence in court

forensic evidence

In the course of preparing for a criminal trial in which the only evidence against the accused was DNA evidence linking him to the victim, I came across a number of useful resources which I share on this page.

Debunking the ‘CSI Effect’

The ‘CSI Effect’ is the phenomenon whereby fact-finders believe that forensic science has the miraculous potential to solve crimes and convict the guilty, based on fictional portrayals of the power of science in criminal justice.

The fact-finder needs to be carefully reminded of the problems with this portrayal of forensic science. Forensic science is concerned with probabilities and rarely, if ever, provides certainty. For example, a reference to a ‘DNA Match’ is not actually a reference to a match, it is a reference to a very high probability that two DNA samples came from the same person.

In addition, aspects of forensic science can be subjective and heavily influenced by the personal views and experience of the scientist expressing the opinion. Those opinions are, in turn, influenced by the background information that the scientist has been given in relation to the case.

It must also be remembered that aspects of forensic science (DNA, for example) are susceptible to contamination not only at the crime scene but also on the journey from the crime scene to the lab and during analysis in the lab. The fact that forensic scientists are sometimes working with items measured in picometres (a picometre is one trillionth of a metre) underscores the need for absolute accuracy in the collection, transmission and testing of samples.

Challenging the Expert Witness

The evidence of an expert witness can, broadly speaking, be broken into two parts – the underlying facts and the conclusions (opinions) drawn from those underlying facts.

It is said that an advocate will rarely, if ever, win an argument with an expert about the expert’s conclusion or opinion. However, by examining the underlying facts it may be possible to find errors or extract concessions which impact the validity of the expert’s conclusion.

The expert witness will also, often, have to make certain basic presumptions about certain matters. For example, a forensic scientist analysing genetic material will often work on the presumption that:

  • The genetic material was not degraded or contaminated before or during collection;
  • The genetic material was collected properly at the crime scene or from the victim or suspect;
  • The sample was successfully transported to the laboratory without interference or contamination.

If any of these presumptions can be shown to be incorrect then it follows that the expert’s conclusion is, to some degree, unsafe.

Useful Resources

European Network of Forensic Science Institutes (ENFSI)


Description: Founded in 1995 with the purpose of improving the mutual exchange of information in the field of forensic science, ENFSI has been recognized as the monopoly organisation (sole voice) in the field of forensic science by the European Commission.

Resources: A wide range of Guidelines on validation, sampling, analysis, and the presentation of forensic evaluation evidence in reports and in court. (Click here for ENFSI Forensic Guidelines page.)

Royal Statistical Society (RSS)


Description: The RSS has a ‘Statistics and Law’ Section which grew out of concerns around the case of Sally Clark and miscalculation of the probability of two cases of Sudden Infant Death Syndrome in a family. They have produced a number of guides to assist legal and forensic practitioners in the interpretation of forensic evidence.


  1. Statistics and probability for advocates: Understanding the use of statistical evidence in courts and tribunals.
  2. Fundamentals of Probability and Statistical Evidence in Criminal Proceedings
  3. Assessing the probative value of DNA evidence
  4. The Logic of Forensic Proof: Inferential Reasoning in Criminal Evidence and Forensic Science
  5. Case Assessment and Interpretation of Expert Evidence

Royal Society of Edinburgh


Description: Easy-to-understand guides or primers on scientific evidence were introduced in UK courts as a working tool for judges and distributed to courts in England and Wales, Scotland and Northern Ireland through the Judicial College, the Judicial Institute, and the Judicial Studies Board for Northern Ireland.


  1. Forensic DNA Analysis: A Primer for Courts
  2. Forensic Gait Analysis: A Primer for Courts
  3. The use of statistics in legal proceedings: A Primer for Courts
  4. Understanding ballistics: A Primer for Courts



Description: Build skills with courses, certificates, and degrees online from world-class universities and companies. Coursera is free to join and offers a wide range of free and paid courses.


  1. Challenging Forensic Science: How Science Should Speak to Court – a free course delivered by faculty at the School of Criminal Justice of the University of Lausanne.
  2. Introduction to Forensic Science – a free course delivered by faculty at the School of Physical & Mathematical Sciences at Nanyang Technological University, Singapore.
  3. DNA Decoded – a free course delivered by biochemistry professors at McMaster University, Canada.

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