DNA Likelihood Ratio Wrong: When a “1 Billion” DNA Statistic Collapsed

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

  • - 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.
2019: Sexual assault – DNA excluded pre-trial →

 4. WHY DNA LIKELIHOOD RATIOS CAN BE WRONG OR MISLEADING

  • - 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.

 5. How STRmix and DNA Statistics Are Calculated

  • - 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.

 6. WHEN DNA LIKELIHOOD RATIOS CAN BE WRONG

  • - Relatives cohabit or frequently visit
  • - Close social contact exists
  • - People share bedding or clothing
  • - Worn clothing and bedding are stored or laundered together

 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.
2025: DNA contamination – challenged – prosecution withdrawn →

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