Question: In the crown court, do people ever question your evidence?

Sue Carney answered on 15 Jun 2011:

Hi janieolver. That’s a really important question so I’m glad you asked that. Yes, in crown court any expert evidence can be questioned and it’s very important that happens for it to be a fair trial.

In England, we have what’s called an adversarial system in our crown courts. (It’s a slightly different system in Scotland.) That means that there are two sides or adversaries: one side arguing for the prosecution and presenting evidence that the the accused person (the defendant) committed the crime, and the other side, the defence, arguing on behalf of the defendant and presenting evidence that he or she didn’t commit the crime but that another sequence of events happened instead.

The trial itself is very civilised (usually!) and everything happens in a particular order. The prosecution present their case first, with various prosecution witnesses giving evidence. Each witness is questioned by the prosecution barrister then questioned again (cross examined) by the defence barrister. When the prosecution have finished making their case, the defence present their arguments by bringing in the defence witnesses. These witnesses are also questioned by both barristers. At the end,the judge sums up both sides of the argument and all the evidence before the jury go out to deliberate on their verdict.

When a forensic scientist gives evidence they may be appearing as a witness for the prosecution or for the defence. In most of my court appearances, I’ve attended as a prosecution witness. However, as a forensic scientist, I am impartial and must present my evidence without any bias towards either prosecution or defence. The scientist’s (or other expert witness) role is to explain how much they think their evidence supports the prosecution or the defence version of events, but not to give an opinion on how likely either of those versions of events are. It’s a very subtle, but a very important difference.

The expert witness is usually asked to confirm who they are and to explain why they are qualified as an expert. That’s very important for the court to know that they can rely on my expert opinion. The prosecution barrister then asks questions which allow me to explain which items were examined in a case, what the purpose of the examinations were, what the results of the tests were and how I have interpreted those results. This all leads to my conclusion which states whether the forensic evidence supports the prosecution or the defence view, and by how much.

The defence barrister is then allowed to cross examine me. He or she might ask questions about what we call continuity or the chain of evidence. It’s important here to be able to demonstrate that the exhibits I’ve tested are the right ones, by having a record of where they were at all times and who has had contact with them between when they were recovered from the crime scene right upto and even after the forensic examination. The defence barrister might also ask questions about the science and sometimes the defence employ their own forensic expert who has had chance to review my findings and might suggest alternative interpretations of the results of tests. (You ask me this question at an interesting time incidentally, because my current work is heading more into the realm of the defence expert, so soon I might be the one suggesting science questions for the defence barrister to ask another expert.)

Sometimes, the judge also asks questions, if he or she thinks any point needs further clarification.

So, the short answer to the question was: Yes! Forensic scientists are always aware that their evidence will be scrutinized in this way, and it’s quite proper that it is too.