Koh Tao DNA evidence & international standards

Koh Tao DNA evidence "shows absolute ignorance of literature of last 20 years & all international guidelines" says leading forensics expert.

CRIME & TECHNOLOGY

Koh Tao DNA evidence & international standards

An international forensics expert has serious concerns about the science behind claims of a '100% match' in the Koh Tao murder case.
10/01/2016
Spectrum
Abridged version of article by Dane Halpin & Nanchanok Wongsamuth

On Christmas Eve, three judges at the Koh Samui Provincial Court found Zaw Lin and Win Zaw Htun guilty of the murder of British tourists David Miller and Hannah Witheridge, sentencing the pair to death.

The ruling relied heavily on the DNA evidence put forward by prosecutors, which claimed a "100%" match and which the court agreed used methodology that was "internationally accepted".

Serious doubts, however, linger over the scientific method and validity of the DNA evidence presented by the police during the trial. 

This was a groundbreaking legal case, the first time a Thai criminal defence team ever challenged police forensic evidence and were granted a retest.

Some Thai DNA experts have even questioned whether the police lab responsible for testing in this case had ISO17025 certification at the time of analysing forensic samples in the Koh Tao case.

Ms. Jane Taupin, one of the world's foremost experts on DNA profiling, who has worked as an internal laboratory auditor and written books on the use of forensic evidence in courtrooms, has provided a thorough critique of DNA evidence in the case which follows.

MUST EXPLAIN STATISTICS 

Crime scenes, Ms Taupin says, are notoriously difficult to gather quality DNA samples from.

Cases like the one on Koh Tao, which deal with mixed samples, are fraught with danger: complex and sometimes unreliable statistical calculations must be carried out to determine the probability that someone other than the accused could match the recovered sample.

Yet no statistical analysis was made available to the defence team or the court.

“The DNA evidence cannot lie,” said police spokesman Pol Maj Gen Piyaphan Pingmuang.

But Ms Taupin is not so convinced.

“It’s quite specific that if you say a particular person has contributed to a DNA sample or crime stain, then you must state the statistical significance. You must. And the ‘must’ is in capital letters and underlined. That is an international guideline,” she said.

“So to say a crime scene stain matches a reference sample without any significance of inclusion, without any statistics, follows no recognised guideline, none at all.”

Instead, the judges were simply told by prosecution witnesses the evidence "confirmed" that DNA recovered from the crime scene belonged to the two accused

Such a claim is not strictly possible to make, says Ms Taupin; instead, a probability ratio must be given.

"Whatever way you want to determine the statistics, they’ve got to be validated in your laboratory, and you’ve got to have them. But Thailand doesn’t have them. Not at all," she said.

"DNA profiling is predicated on statistics, that’s the whole point. You don’t just say it’s a match — it’s not fingerprinting … You need to give significance to that match."

Forensic scientists must be acutely aware of the potential flaws in their experiments, and also explain these potential shortcomings to the court, Ms. Taupin says.

Ms Taupin said the whole purpose of analysing DNA profiles is not about certainty to begin with — it’s about statistical probability...

CASE DOGGED BY PUBLIC BLUNDERS

From the moment the bodies of Miller and Witheridge were discovered on Sairee Beach on Sept 15, 2014, police found themselves under enormous pressure to resolve the case quickly.

But the investigation was dogged from the outset by a series of very public blunders, including concerns the crime scene had not been properly secured and could have been contaminated.

It soon emerged that CCTV cameras in the vicinity of the crime scene were not working, while cameras near the pier — the small island’s sole entry and exit point — were never inspected.

Thai nationals were ruled out almost immediately as suspects, and police quickly declared the island’s Myanmar migrant population was most likely responsible.

Police announced that DNA samples showed the perpetrators were Asian, even though DNA profiling is not able to determine race — a fact later acknowledged by police forensic experts in court.

DNA EVIDENCE

On Sept 16, the day after the murders, 11 pieces of evidence recovered from the crime scene, along with DNA samples taken from 14 suspects, were sent to a police forensics lab for testing.

One of those pieces of evidence was the murder weapon, a garden hoe. An initial test found traces of human blood, which proved to be that of Witheridge. But no DNA was recovered.

Almost a year after that test was conducted, the hoe was retested — this time by a team led by Dr Pornthip. It found traces of DNA belonging to at least two people, but which did not match with the profiles of the accused.

This obviously raises the question of who used the hoe and left their DNA on the hoe and whether they were the ones who used the hoe as a murder weapon. Why did the initial tests by the police not reveal this DNA?

A key part of the prosecution's case was what they said was a "match" between DNA samples found on Witheridge’s body and the two defendants.

Defence lawyers had asked police to hand over the DNA samples that the police used to make this match, for independent verification, but were told there was nothing left of the evidence for retesting.

MIXED DNA SAMPLES: REQUIRE SPECIAL HANDLING

When a crime scene sample arrives at the lab for testing, forensic scientists are forced to make informed guesses about how many people might have contributed to the mixture of DNA recovered from a piece of evidence.

The more possible DNA contributors to a piece of evidence, the harder it becomes to reasonably link the evidence to any individual person.

The scientists will not analyse entire genomes; instead they look at just a small part. How many areas are tested will differ by country, but in Thailand scientists will check for commonalities at 16 specific locations, which is considered acceptable by international standards...

MIXED DNA SAMPLES: FALSE LINKS EASY

In the Koh Tao case, the most compelling DNA evidence was that recovered from semen sampled from Witheridge’s body, which contained at least three people’s DNA — the victim and what police said were the two accused. Yet Ms Taupin said it’s not clear how mixed sample statistics were applied to those samples.

... the number of potential matches vastly increases... and therefore... in mixed samples it is easy to create false links between an accused and a crime scene.

...During the Koh Tao trial, the court was told samples were found which match the profiles of the suspects in all 16 locations.

But no statistic was offered to show how many other people in the population might also have matched, which Ms Taupin said defied international guidelines...

DNA ALONE NOT ENOUGH FOR CRIMINAL CONVICTION

Given the nature of mixed sample DNA analysis, she is particularly critical of claims by multiple police forensics experts that their lab results guaranteed the identity of the perpetrators "100%".

She said there is no international standard that recommends the conclusion of identity from a forensic DNA profile, no matter how many areas on the DNA molecule are examined.

It is for this reason, she said, that DNA profiling alone is not enough to form the basis of a criminal conviction. "It’s just one piece of the circumstantial case. It’s a scientific test, that’s all," she said.

PEER REVIEW ESSENTIAL

Ms Taupin said there are many areas during the DNA profiling process where mistakes can be made, which is why the availability of data for peer review is essential.

“There are many ways that you can misinterpret a particular value as being an actual ‘true’ allelial result, and it’s not actually so,” she said.

Ms Taupin pointed to potential mistakes in the limited data presented to the court in the Koh Tao case.

 In the one-page table, an extra "allele value" — basically an extra Scrabble letter — was found in the DNA sample taken from Witheridge’s breast which did not match either of the accused.

"They [the prosecution] said, ‘Oh well, that doesn’t matter because the other components matched the accused,'" Ms Taupin said.

"But it does matter, because if there’s an extra value in there, and if the two accused contributed, then a third person must have donated."

She said there were also a number of components in the retrieved semen samples, particularly in the anal swab recovered from Witheridge, that were a mixture of at least two people and possibly three.

But the steps to achieve all this were not provided.

DON'T JUST TEST UNTIL YOU GET A MATCH  

Ms Taupin also raised concerns about the mass collection of DNA that took place on Koh Tao as police desperately searched for suspects in the days after the crime.

"If you test a lot of people, you don’t just test until you find that someone matches, because how many other people might match, how many other people that have left the island might match, or how many people in Thailand might match, or how many people in the United Kingdom might match.

“A fallacy, and that’s what I think happened, is that they’ve tested people until they’ve found people that had components that were in the mixture.”

If that is the case, she said, it would mean police have not looked at the whole concept of mixture statistics.

ACTIONS SHOW ABSOLUTE IGNORANCE OF LITERATURE & GUIDELINES

In her final analysis, Ms. Taupin does not even give the police a passing grade on the handling of DNA evidence in the Koh Tao murder trial.

This raises two obvious questions. First, how can this flawed evidence legitimately be used to support a sentence of death?

And second, how many similar criminal cases have there been in the past, that were not in the public spotlight like the Koh Tao case, and passed by silently without any challenge at all to flawed forensic methods used by the police?

Ms. Taupin concludes, “how they could have ignored mixture statistics defies belief, and shows an absolute ignorance of the literature of the last 20 years and all international guidelines.”

“One particularly disturbing aspect was that it said there was saliva on the right breast of the deceased, but there was no test for it. It was just assumed to be saliva. I mean where did they get that from?”

http://www.bangkokpost.com/news/special-reports/821072/unlocking-the-dna-of-doubt

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