DNA Likelihood Ratio Wrong: When a “1 Billion” DNA Statistic Collapsed
DNA evidence can be overstated and misapplied in criminal cases.
Phrases like “1 billion times more likely” sound decisive. But a DNA likelihood ratio error can occur if the wrong comparison is made, or if important case information is ignored. These types of problems with DNA likelihood ratios can produce unreliable DNA statistics.
This case explains how a 1 billion DNA likelihood ratio used in an actual prosecution was shown before trial to be wrong.
1. The Prosecution Allegation
- - It was alleged that Mr Smith sexually assaulted his partner’s teenage daughter.
- This was said to occur in his bed.
- A DNA profile was obtained from Mr Smith’s bedsheet.
- The prosecution relied heavily on a statistic reporting the complainants DNA.
- In this case, the statistic was wrong.
2. The Defence Position
- - Mr Smith denied the allegation.
- He was in a sexual relationship with the complainant’s mother.
- Mr Smith and the mother regularly slept in the bed.
3. What a DNA Likelihood Ratio Actually Means
DNA reports might say “The DNA profile is 1 billion times more likely to be obtained if the complainant is a contributor.”
- - The DNA likelihood ratio can be as high as 100 billion.
- - This does not mean there is a 100 billion to 1 chance the allegation occurred.
- It does not mean there is only 1 person in every 100 billion that have the same DNA profile.
Important note: There is almost always other DNA work conducted by the laboratory which is not contained in the expert report. The DNA case file must be reviewed to understand what both the reported and unreported results can properly support.
In this case, the report failed to consider the actual living circumstances of Mr Smith and his partner. DNA likelihood ratio issues also occur with inaccurate ethnicity database selection.
4. WHY DNA LIKELIHOOD RATIOS CAN BE WRONG OR MISLEADING
The likelihood ratio can be wrong when a family member may be the true DNA contributor. In this matter:
- - Mr Smith and his partner shared the bed.
- The complainant was the daughter of Mr Smith’s partner.
- The DNA on the bedsheet may be from Mr Smith’s partner, not her daughter.
The DNA report did not consider Mr Smith’s partner.
The likelihood ratio was therefore invalid.
5. How STRmix and DNA Statistics Are Calculated
STRmix is computer software used to read DNA profiles from multiple people:
- - It compares genetic markers from DNA sample to another DNA sample.
- - It generates a likelihood ratio based on comparison.
- - In this case, STRmix did not make an error.
The analyst failed to report the complainant’s mother as a possible DNA contributor to the bedsheet.
6. WHEN DNA LIKELIHOOD RATIOS CAN BE WRONG
Criminal allegations arise where family members, partners, and relatives are intertwined. These factors must be evaluated alongside any DNA statistic.
Care is advised when:
- - Relatives cohabit or frequently visit
- - Close social contact exists
- - People share bedding or clothing
- - Worn clothing and bedding are stored or laundered together
Likelihood ratio issues may occur in these circumstance and warrant challenge.
7. HOW TO CHALLENGE DNA STATISTICS
- - Errors are rarely established by reading the DNA report.
- We conducted a detailed scientific review of the DNA case file, including the STRmix extended outputs.
- We established that the mother was more likely the source of the DNA than the complainant.
- Once the appropriate comparison was presented, the prosecution statistic collapsed.
- The prosecution withdrew the DNA evidence following receipt of our report.
When DNA evidence looks conclusive but isn’t
- - It is a red flag if you deny the allegation, but the DNA report appears statistically overwhelming.
- - Challenging DNA likelihood ratios usually requires a DNA expert likelihood ratio review of the statistics used in the report.
Do you need a DNA expert?
A criminal lawyer and a DNA expert are two distinct specialities.
DNA statistics can appear simple within an expert report but are highly complex beneath the surface.
It is not possible to determine whether a DNA likelihood ratio has been properly calculated - or whether the correct comparison has been used - without a detailed scientific review of the DNA case file
Helen Roebuck is a strongly credentialed and highly regarded forensic DNA expert, trusted within both the legal and scientific communities. She is the most prolific and judgment-published independent DNA expert witness in Australia
Having reviewed DNA evidence in thousands of indictable matters over the last 25 years including homicide, sexual assault and drugs importation.
Accepting simple and complex Legal Aid and privately funded briefs across Australia, New Zealand and select international jurisdictions.
Published judgments:
WA v Piccioni [2025] – WA District Court
R v Murphy [2024] – NSW District Court
LGBTIQ Hate Crimes Inquiry [2023] – NSW
R v Granaghan [2022] – N Ireland Crown
R v Perre [2021] – SA Supreme Court
R v Ke [2019] – NSW District Court
R v TAL [2019] – QLD Supreme Court
R v Haouchar [2025] – NSW District Court
R v Ahmad [2024] – NSW District Court
Evans v R [2023] – NSW Supreme Court
R v Tsakirios [2021] – NSW District
R v Kabbout [2020] – NSW District Court
In many matters, a short scientific review of the DNA casefile is enough to determine whether the DNA work conducted by the lab genuinely supports the allegation or whether the DNA results support an alternate explanation of events.
