3 Likelihood ratio calculations

Would you be able to explain likelihood ratio calculations in lay terms? Continue reading to see how it could be explained.

In simple terms, the expert is communicating that

1. DNA that matches the suspect’s DNA is on the bat
2. the chances of this DNA profile being obtained from someone other than the suspect is very small
3. on the basis of (1) and (2) it is more likely that the suspect’s DNA is on the bat than someone else’s DNA.

However, this does not mean that the suspect actually handled the bat. So, while the DNA evidence may answer the first and second question (Is there DNA on the bat? And who’s DNA is that most likely to be?), it cannot reliably answer the third statement (How the person’s DNA came to be on the bat and this type of evidence, on its own, cannot address the offence level question as to whether the suspect is the person who committed the alleged crime).

During the cross-examination, the defence lawyer may not contest the answer to the first or second question. However, the answer to the third statement will and should be of high interest. Remember the issues raised in Week 2.

Research has shown that it is possible for a person to handle an item and leave very little to no DNA behind, or for a person’s DNA to be transferred to an item or surface without direct contact. This is a process known as secondary or tertiary transfer (DNA Judicial Primer; 2025, van Oorschot et al, 2019).

A good line of questioning might be who’s DNA is on the bat and where (location) on the bat the DNA was recovered from (for example the victim’s DNA would be recovered from the blood stained area and the suspect’s DNA might be recovered from the handle); how likely it is that the suspect’s DNA was transferred to the handle of the bat through direct contact or via a different transfer mechanism and then remained (persisted) on the bat. What are the chances of such contact happening either during, before or after the alleged incident, in such a busy gym? These questions challenge the relevance, evaluation and weight of the DNA evidence.

Remember in Week 2 that the suspect stated that he was a member of the gym, frequented the locker room, but had nothing to do with the alleged attack. If the suspect handled the bat at any time, his DNA could have been left on the handle. It could also be transferred to the bat from the locker room environment, without direct contact although this would be less likely. Equally, if the suspect handled the bat when using it to assault the victim his DNA could be present. In both scenarios, the presence of a DNA match (the forensic evidence) is equally likely and plausible, however it will be conditioned also on what is known about the persistence of DNA on baseball bat handles and what impacts that (time since handling, pressure, humidity and environmental conditions etc).

The DNA evidence doesn’t answer how the DNA got to be where it was recovered from (the handle of the bat), only that it was there. The DNA evidence alone in this case is insufficient to answer the question of whether the suspect committed the crime.