News November 23, 2021

By Forensic Access

In Part 1 of this news item we outlined the basis for Source Level DNA Reporting and some of the key considerations required when evaluating mixed DNA results with specialist statistical software.

However, it is also important (arguably more so) to consider the potential implications when there is an absence of statistical analysis in a DNA report. Additionally, the second area we will cover is how critical it is that the correct version of events is considered by any scientist when evaluating the DNA results at activity level. This is potentially the most crucial aspect of any defence review and is based on the accuracy of the information available.

Absence of Statistical Analysis

What if a report contains no statistical evaluation or, more importantly, what if a finding is not suitable for statistical evaluation?

There are occasions where the absence of statistical analysis leads to questions about the accuracy of the findings and whether or not we can draw conclusions from the results. This is where language and terminology of the report comes into play.

A lay person reading the report may not discern the same meaning as an experienced scientist, leading to a misunderstanding of the overall evidential significance of the report. For the scientist to tell the court that the defendant might have been a contributor to the mixed profile is unbalanced and in favour of the prosecution, unless the scientist adds that the defendant might not have been a contributor.

Effectively, this means that the court has no further information than if the DNA profiling had not been carried out. The finding is evidentially neutral and does not add probative value to the case.

The question at this point becomes is the finding suitable for specialist statistical evaluation, has this just not been done yet, or is it simply not suitable for such a calculation to be completed. If it is the latter, how does this impact the case?

Activity Level Reporting: “How did the DNA get there?”

Once it has been established who any detected DNA could have come from the next question is typically how did the DNA get there.  This is where the reporting of the result at Activity Level is required.

The first stage of activity level reporting is to try and understand from what body fluid the DNA could have come from.  For example did the DNA come from blood, semen, saliva or is it ‘cellular material’ from skin cells, for example.  If we have a detected body fluid such as blood in our sample then it may be easy to attribute the DNA to having come from this blood, as we would have a high expectation of obtaining a good quality DNA profile from a source of blood staining.  However, as we can now produce good DNA profiles from very small amounts of DNA, we need to evaluate results carefully, especially when dealing with ‘touch’ DNA samples and mixed DNA results.

For all DNA samples tested we would first consider:

  • What area has been sampled and how was the sample taken?
  • What tests have been employed (for example was the sample tested for blood)?
  • How much DNA is present?
  • Would we be happy to say the profile derived from that particular stain?

This information helps us to build up a picture of the types of sources from which the DNA could have derived from, as well also the types of scenarios in which we might expect that DNA to have been transferred or deposited.

The ultimate question we are usually faced with is ‘how was the DNA deposited?’.  This is usually framed around two propositions as to what happened, one provided by the prosecution and the other by the defence.  We will then consider the information available in the context of the above propositions to try to understand which is more likely.

The evaluation of the propositions typically becomes more difficult as the amount of DNA detected decreases, or if we are dealing with samples involving mixtures of DNA from more than one individual.  There is a significant amount of research data now available in relation to issues such as the transfer and persistence of DNA.  However, it is still ultimately down to the scientists experience and knowledge in this area as to how they evaluate these issues and ultimately report the significance of their findings. 

It is also the case that should the information available regarding the circumstances change, or indeed if either or both the propositions put forward by prosecution and defence change, this can significantly affect the final evaluation of the findings.  Therefore, it is always advisable to ensure as much information as possible is available to the scientists to aid in their evaluation, and should you be aware information has not been put forward and is available, you should speak to a DNA reporting scientist to assess whether a further review is worthwhile.


Forensic DNA Expertise

Forensic Access boasts a world-class team of forensic scientists operating in bespoke facilities. All forensic work is carried out to the highest quality standards and we are always available to offer advice, discuss your concerns and determine whether a full case review is required.

All defence work is handled by dedicate Casework Managers who provide end-to-end support and streamlining. Direct access to our team of scientists helps barristers and solicitors to prepare a more effective defence strategy, and all expert witness reports are thoroughly documented and reviewed.

To find out more about our forensic services Tel: 01235 605794 to speak with a member of our team or fill-in our online contact form.