DNA Evidence Interpretation Variability Study

DNA evidence has become a crucial tool in forensic investigations, often serving as a powerful means to identify suspects, exonerate the innocent, and provide closure to victims and their families.

However, the interpretation of DNA evidence is not always straightforward or perfect, which leads to inaccuracies and variations in the interpretation process. These can have significant implications for the reliability and admissibility of this evidence in court. This article explores the extent of interpretation variability in DNA evidence, the factors contributing to it, and the strategies employed to reduce it.

Key Statistics on Interpretation Variability

Inter-Laboratory and Intra-Laboratory Variability

A study funded by the National Institute of Justice (NIJ) highlights the significant variability in DNA evidence interpretation across and within forensic laboratories.

• The study involved sending complex DNA mixture samples to different laboratories for interpretation, and notable differences in the results were found
• The inclusion of a known reference DNA profile had a marked positive effect on the interpretability of DNA mixtures
• Some laboratories struggled with interpreting three-person mixtures even with reference samples, indicating the challenges in analyzing complex DNA samples

Impact of DNA Sample Quality and Quantity

The quality and quantity of DNA samples can significantly influence the interpretation of DNA evidence.

• Low quantities of DNA and degraded DNA samples can affect the ability to obtain reliable DNA profiles. The presence of inhibitors can also impact PCR amplification, leading to partial profiles or allele dropout.
• A study found that the largest variation in DNA yield was observed between centers, accounting for up to 16.9% of the explained variation, suggesting that variability can also stem from pre-analytical factors.
• Environmental conditions, such as temperature, humidity, and UV light exposure, can affect the persistence and degradation of DNA on different surfaces, further complicating the interpretation of touch DNA evidence.

Human Factors in Interpretation

Human factors, including cognitive biases and the experience level of forensic analysts, play a significant role in the interpretation of DNA evidence.

• Studies have shown that forensic examiners’ accuracy increases with more time given for comparisons, indicating the importance of analytical processing in addition to holistic processing
• The implementation of probabilistic genotyping software (software that matches up DNA samples to persons of interest) has been suggested to reduce human interpretation errors, but it also requires careful validation and understanding of its limitations
• Incorrect assessments of the number of contributors to a DNA mixture have been shown to affect statistical analyses in some cases, although they do not necessarily imply inaccurate interpretations or conclusions

Contributing Factors and Strategies for Reduction

Several factors contribute to interpretation variability in DNA evidence, including the complexity of DNA mixtures, the methodologies used for analysis, and the experience and training of forensic analysts.

Strategies to reduce interpretation variability in DNA evidence include standardizing protocols, providing comprehensive training and certification for forensic analysts, using probabilistic genotyping software, and implementing quality assurance measures such as proficiency testing and peer review.