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“Experience is the name that everyone gives to their mistakes” 1

By Professor Allan Jamieson and Dr Georgina Meakin, both of The Forensic Institute

Experience and expertise are not the same. From a scientific perspective, expertise is grounded in knowledge of the scientific method and the facts and inferences gained from structured, controlled experiments. Experience is the sum of the information that we have gained, however we have gained it. It may help us make decisions, but it does not necessarily follow that experience leads to correct inferences.

“Clearly we learn from experience but it is the way that we learn that distinguishes the scientific from the unscientific”.[1]

“In short, experience (and data!) is useful in providing high quality forensic science support to a case; but it is anathema to deciding what the results MEAN once the analyses have been concluded. We use our EXPERIENCE to guide us in properly performing the most specific and discriminating tests on the most relevant evidence, and we use DATA to decipher the meaning of the results, form conclusions, and put limits on those conclusions (quantitate the uncertainty).”[2]

Just as others appear to be waking to the problems of the lack of science in forensic science, recent judgements from the Appeal Court in England and Wales have caused concern in the scientific world about the legal system’s understanding of science and scientific expertise. When the Appeal Court states,

“...we do hope that the courts will not be troubled in future by attempts to rely on published work by people who have no practical experience in the field and therefore cannot contradict or bring any useful evidence to bear on issues that are not always contained in scientific journals. There are plenty of really experienced experts who are available and it is to those that the courts look for assistance in cases of this kind.”[3]

the scientific response is sometimes blunt,

“… the courts should be more troubled by "really experienced experts" (what counts as really experienced?) than by attempts to rely on actual data.”2

or more guarded, but nevertheless concurring,

“Recent legal cases in the UK may have potentially begun to move us into what is an uncomfortable arena, where suggestions that years of practical experience are more important to the courts than sound science. …

While this implies that experts have an awareness of the application of the science, it fundamentally requires the provision of an opinion firmly routed in sound science.”[4].

That ‘sound science’ should be the bedrock of forensic science is well recognised,

“The most important piece of information to take away is that forensic science is primarily a job for a scientist. The science comes first, the forensics comes later.”[5]

Science is based on published papers that can be subject to the scrutiny of the entire scientific community (and anyone else for that matter). The National Academy of Sciences of the US recently produced an extensive report on the forensic sciences. It states,

“The simple reality is that the interpretation of forensic evidence is not always based on scientific studies to determine its validity. This is a serious problem. Although research has been done in some disciplines, there is a notable dearth of peer-reviewed, published studies establishing the scientific bases and validity of many forensic methods.”[6]

An example that well illustrates the apparent division between the Appeal Court and the scientific community generally is that of DNA transfer; an issue of increasing importance in criminal cases.

Current DNA profiling technology enables invisible amounts of cellular material that cannot be attributed to a known body fluid such as blood, semen, or saliva - so-called ‘touch DNA’ – to be profiled. However, the cells from an individual containing DNA can be found on an item through direct or indirect transfer. Direct transfer occurs by a person actually touching the item. Indirect, or secondary, transfer occurs when cells from an individual end up on an item by being carried on an intermediate person or object. It’s like dirt, oil, bacteria, or any one of the innumerable things that get moved around inadvertently by being transferred from person to person or person to object, or object to person. Clearly, at such microscopic levels (you can fit about 10,000 cells on the head of a pin and some claim to be able to profile about 10 of them), there are many and varied ways that this could potentially happen. In criminal cases, it can be of the greatest importance to understand how DNA that could have come from a suspect actually came to be on an item. Frequently, the issue is the last handler of the item, such as a weapon or item of clothing.

Remembering the central role played by research in underpinning a scientific opinion, a review of the scientific literature on DNA transfer in 2007 concluded that,

“… the literature so far indicates that:

1. Where there is a known single habitual wearer, that person tends to be detected as the major source of DNA on a garment; minor profiles may also be detected from individuals with whom the habitual wearer has had close contact as well as from unknown sources.

2. The examination of evidence for handler DNA can reveal DNA of people who have, or have not, handled the item; the stronger profile may, or may not, be the person who last handled the item; An inference of direct contact between an individual and the item may or may not be supportable, depending on the circumstances of the case.”[7]

In other words, we did not know enough to make any sensible scientific judgements as to how DNA came to be on an item.

Jump to 2010 and the Appeal Court of England and Wales in 2009 and 2010 stated,

“…However, in our view, a forensic science officer with scenes of crime experience such as [VT] can properly use knowledge of the scene of the crime and the other agreed circumstances to evaluate those possibilities by reference to her experience and the scientific research that has been undertaken. …

we accept that [VT] could properly give admissible evidence evaluating the alternative mechanisms of primary and secondary transfer.”[8] [our underline]

and

“[Dr C] was able to put forward on the basis of his experience a sufficiently reliable scientific basis for a forensic science officer to give evidence of the evaluation of the possibilities of transfer”4 [our underline]

Two strands then, experience and scientific research. Notwithstanding the views expressed above2 on the proper limitations of what scientific experience can deliver as regards opinion, what does experience permit the scientist to opine on DNA transfer?

Consider how a forensic scientist gains experience. They receive exhibits into the laboratory, they discover what’s on them, and they inform the client of the results. Frequently, the underlying hypothesis is that touching, or direct contact, is a more likely scientific explanation for the finding of a DNA profile on an item than indirect contact (such was the position of the Crown in Reed and Reed7). This to the extent that it may be described as providing ‘extremely strong’ support for direct versus indirect transfer. In our view, such an opinion on DNA transfer is not supportable based on case experience or on the available scientific research.

We have recently been involved in a case where we challenged the ‘case experience’ cited to support the opinion on DNA transfer. In short, the Crown could provide no data from casework that could be used to determine the chances of finding a DNA profile from the different modes of transfer, given that by definition the cause of the profile is unknown. There is no scientific means, from the DNA profile alone, to know whether the item was actually touched by the defendant; especially if the defendant denies any involvement.

The logic used to derive the opinion appears to be, touching causes the finding of a DNA profile, we find a DNA profile, therefore this person has touched it. This is similar to; colds cause coughing, this person is coughing, hence this person has a cold. Coughs can of course be caused by many other conditions. To know whether it is more likely that the cough is caused by a cold requires more information; for example, the location and history of the patient. The analogous situation in DNA transfer is; direct transfer (touching) causes the presence of a DNA profile, there is a DNA profile, hence this profile was caused by direct transfer. The flaw in this logic is such that this begs the question that needs to be answered because it assumes that touching should cause transfer of DNA. Direct transfer may be a more obvious cause, or may appeal intuitively, but can only be assessed by sufficient properly controlled studies, not from casework experience.

The Appeal Court did take a view on the contribution that published research could make to the assessment of the different modes of DNA transfer. Although

“… scientific knowledge is incomplete, the science is sufficiently certain for that evidence to be admissible in all the circumstances of this case.”7

They even quoted two scientific papers, both of which even the Court acknowledged were preliminary and required further work (in fact, one had failed to confirm some of the findings of the other).

So what does the scientific literature have to say in 2010?

From their observation that the majority of high level mixed DNA profiles obtained from beneath fingernails were associated with recent intimate contact, Dowlman et al. (2010) comment,

“Further studies into the transfer of DNA transfer [sic] after digital penetration, will hopefully address this issue.” [9]

In an investigation into the secondary transfer of salivary DNA from skin to fabrics, Kenna et al. (2010) write,

“It should be noted that this study was limited in its scope. A more detailed examination of all the factors that affect secondary transfer is justified.” [10]

A study that looked at a number of those factors, such as the types of biological substance, surface, and contact involved in transfer, concluded that,

“This paper is a contribution to our better understanding of secondary DNA transfer. Having established some baseline information using easily manipulated relevant biological samples such as blood and saliva, one should consider investigating other biological substances, substrates and conditions.”[11]

These papers not only demonstrate the requirement for further research into DNA transfer but also begin to illustrate the wide range of factors that can affect transfer. Based on the published research, a complete understanding of secondary DNA transfer is still far off and we are no further forward than the conclusions drawn by Rudin and Inman in 20076. Consequently, Goray et al. (2010) comment,

“The lack of knowledge relating to secondary DNA transfer can limit the acquisition of useful genetic profiles and restrict the ability of investigators, as well as the judiciary, to make reasonable evaluations of the likelihood of alternative crime scene scenarios.” 10

This has been acknowledged by some forensic practitioners who note that, when a low level DNA profile is recovered from an item, no inferences about the means of deposition can be made[12].

Yet the Appeal Court still found,

“that the science is sufficiently reliable for it to be within the competence of a forensic science expert to give admissible evidence evaluating the possibilities of transfer in DNA cases where the amount is over 200 picograms and when there is a sufficient evidential basis from the profiles and other material, as there was in this appeal, for it to be done.”7

It is obscure, absent any research on the amounts of DNA involved in transfer, how the amount of DNA could provide any basis for a distinction on the reliability of opinions on transfer.

In our view, the legal system’s reliance on experience in matters of science is misplaced as the example of transfer illustrates.

The use of Courts as an effective means to settle arcane scientific matters was addressed by the National Academy of Sciences. They were unconvinced.

“For a variety of reasons—including the rules governing the admissibility of forensic evidence, the applicable standards governing appellate review of trial court decisions, the limitations of the adversary process, and the common lack of scientific expertise among judges and lawyers who must try to comprehend and evaluate forensic evidence—the legal system is ill-equipped to correct the problems of the forensic science community. In short, judicial review, by itself, is not the answer.”

The academic lawyer, Professor David Faigman (quoted in 2), says,

“Many witnesses have learned to invoke experience as a means of circumventing the responsibility of supporting an opinion with hard facts. For the witness, it eases cross-examination. But it also removes the scientific basis for the opinion.”2

Rudin and Inman say,

“…this Court makes the critical mistake of redefining science, rather than accepting any of the more standard definitions proffered by the field itself. … This reliance stems from courts asking question to which they need answers, and laboratory workers willing to oblige them with answers that are not born of the long scientific tradition of observation, documentation, research, publication, peer-review, and replication. A more prudent response from science should be, I don't know the answer to that question; we must either do some research (open to review and critical evaluation), or courts must accept that, at the present time, science cannot provide a reliable answer to that particular question….

We need to back-peddle [sic] from the idea that bald experience is an acceptable substitute for experimental data.” 2

It would appear that the UK is in danger of heading in the opposite direction.

1 Oscar Wilde, Lady Windermere’s Fan. 1892



[1] Evett, I W, Expert evidence and forensic misconceptions of the nature of exact science. Science & Justice, 1996 36(2), 118-122.

[2] Rudin, N. & Inman, K. The CACNews, 4th Quarter 2010

[3] R v Weller. Neutral Citation Number: [2010] EWCA Crim 1085

[4] Editorial, Science and Justice 50 (2010): 111–112

[5] Prof Robert Forrest, President, Forensic Science Society, New Scientist 16th June 2007, p. 61

[6] National Academy of Sciences of the US report; Strengthening Forensic Science in the United States: A Path Forward. 2009

[7] Rudin, N. & Inman, K. The CACNews, 3rd Quarter 2007

[8] R v Reed & Reed. Neutral Citation Number: [2009] EWCA Crim 2698

[9] Dowlman, E. A., Martin, N. C., Foy, M. J., Lochner, T. & Neocleous, T. Science and Justice 50 (2010): 64-71

[10] Kenna, J., Smyth, M., McKenna, L., Dockery, C. & McDermott, S. D. Journal of Forensic Sciences doi: 10.1111/j.1556-4029.2010.01520.x

[11] Goray, M., Eken, E., Mitchell, R. J. & van Oorschot, R. A. Forensic Science International Genetics 4 (2010):62-67

[12] Caragine, T., Mikulasovich, R., Tamariz, J., Bajda, E., Sebestyen, J., Baum, H. & Prinz, M. Croatian Medical Journal 50 (2009): 250-267

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