Need for statistical analysis in forensic DNA profiling – A legal perspective

As discussed in the earlier lessons, forensic DNA profiling examines certain portions or loci of the nuclear DNA and not the entire nuclear DNA. Therefore, forensic DNA profiling is probabilistic in nature and all results must be submitted along with statistical analysis. Legal practice around the world has accepted that statistical interpretation of DNA results in necessary, and any DNA report is deemed incomplete without it.

In India, the 185th Report of the Law Commission of India, clearly states that “if the samples match, that does not mean the identity is conclusively proved.” The report further explains that statistical analysis must be carried out in forensic DNA profiling cases, therefore clearly establishing a link between scientific accuracy and legal requirements.

Case law across jurisdictions also clearly establishes that statistical analysis is a requirement for forensic DNA reports. In India, in the case of Premjibhai Bachubhai Khasiya v. State of Gujarat (2009), the Gujarat High Court noted that a simple match without random occurrence ratio cannot be considered as “conclusive proof”. In other jurisdictions, courts have refused to admit reports submitted without statistical analysis. In State v. Cauthron (1993), the Supreme Court of Washington held that the trial court incorrectly admitted the testimony of a “match” as it was not accompanied by probability statistics. Further in State v. Hummert (1994), the Court of Appeals took note of the error in admitting testimony of a ‘match’ in the absence of generally accepted population frequency statistics. Similar observations were made by the court in Robert James Brim v. State of Florida (1997), where it was held that a match without any statistical analysis may be deemed “meaningless”. In the UK, in Regina v. Doheny & Adams (1996), the court clearly states that a match between “crime stain and the sample from the suspect does not prove that both have originated from the same source.”

Courts around the world have continuously engaged with the science behind forensic DNA profiling process, before determining the weight of such evidence in a case. In Bokolo v S (2013), the Supreme Court of Appeal of South Africa explains the science behind forensic DNA profiling while considering the competing testimonies of the prosecution and the defence DNA experts in that case. The court also noted the probabilistic nature of forensic DNA evidence and held that the weight of a DNA inclusion is dependent on the probability of the match. Therefore it is imperative that statistical analysis must be a part of scientific practice and legal discourse, to ascertain the admissibility and weight of forensic DNA evidence.