Finding Fibres in Low Numbers
Fibres Deep Dive Pt 7. Secondary transfer by the numbers and Tiernan's first law of forensic fibre research.
Introduction
So we have a body of knowledge that tells us that thousands of cotton fibres transfer onto a garment at time zero and that generally speaking, hundreds of polyester fibres do the same. Whilst we don’t know precisely what the situation is regarding other types of fibres1, we can make generalised estimates based on what we have learned in relation to cotton and polyester and how well a garment sheds its fibres2 .
I suspect that as the knowledge grows of the decades to come, that the general consensus will form that for a garment that sheds its fibres “well” that the number of fibres transferred at time zero will be at least in the several hundred and that is true irrespective of the duration or intensity of that initial contact.3
We also have a body of knowledge which tells us that generally speaking, how remarkably unlikely it is to find fibres on one garment that could have come from another at random, but that when they are found, it is in low numbers. This is what Ken Wiggins et al4 said:
...where alternative sources may be responsible [aka the random other source] are when there are low numbers of matching fibres (less than five)…
The authors gave us a definition of a “low number” being five or less, but really there isn’t much by way of data that helps us scientifically define this threshold. The reality of the target fibre studies is that the number of surfaces that require sampling is so large to generate even a single random occurrence of a single tiny population of target fibres, that the sampling population would have to be increased by several orders of magnitude to even approach something which yields sufficient data generated from random finding of matching fibres that approaches any kind of statistical significance.
Tiernan’s first law of fibres research5 is that you have to get poor eejits6 like myself to sit under a microscope and search for target fibres on the surfaces of thousands of garments, rendering the pursuit of such a statistically valid threshold merely another exercise in forensic fibre examination which is possible to achieve in theory but not in practice7.
This doesn’t mean that Wiggins et al are wrong about 5 fibres or less, and the studies do show that when random matches are found, it is in certainly single digit type low numbers, but it’s a different matter entirely to engage in the well known fallacies and argue that because low numbers of fibres are found, that makes the finding of the fibres more likely to be the result of a “fortuitous match” aka a random event, than contact with the precise garment that is in the hands of investigators.
IMO the results from the target studies are strongest on two points:
That by far the likeliest outcome of a search of a random garment for a target fibre is that no target fibres will be found at all.
That finding of large numbers of target fibres on a garment is so rare that no target fibre study has ever managed to find more than 10 on single garment8
….and IMO the target fibre studies are weakest in determining what the number of fortuitous fibres found on any one surface at random actually means.
The “hidden nugget” in the target fibre studies that is the most telling IMO, strikes at the heart of transfer and persistence and human behaviour. The target fibre studies tell us, that in the rare event that target fibres are found on a surface at random, they must either:
have been deposited by direct contact with a source that happens to be made from the same fibres as the garment in the hands of investigators, and suffered such significant exponential loss over a period of time that the populations of hundreds or thousands were lost in the intervening period, leaving 5 (or fewer) behind, or
that the 5 fibres (or fewer) were recently9 deposited on that surface through a mechanism other than direct contact with a source that happens to be made from the same fibres as the garment in the hands of investigators.
Secondary Transfer
Clothing
I always loved this paper10 . It had it’s limitations, every paper does, but it was so well designed and so simple, delivering the simplest of powerful messages.
These were the messages:
Secondary transfer from clothing to clothing involves very low numbers of fibres.
When it did occur, only 10% of the fibres that had been transferred to a garment were subsequently transferred onto the next surface during contact. Only 5% of the transfer experiments resulted in more than 5 fibres transferring secondarily.
Most fibres transferred secondarily are lost within the first hour
Knowing what we know about persistence, this makes complete sense. If only small numbers of fibres are transferred to a garment at time zero (5 or less) then they can’t possibly hang around for long if the forces they are subjected to in the first 2 - 4 hours are so strong that 80% of them are likely to be lost. 80% loss of a population of 5 fibres only leaves 1 remaining.
This is such a profound finding that it’s significance seemed to escape the authors attention and to this day, IMO, hasn’t been properly articulated in the fibre literature. It provides the scientific basis for the value of looking for “linking fibres” or more properly, “Leitspuren”.11 , but more on that in the case study to come at the end of the deep dive.
What it really means is that the thousands of populations of fibres in low numbers, that are found on a garment at any moment, are likely to tell us far more about recent contact with a complex textile environment than a comparatively one dimensional interpretation of the numbers of fibres from a single item of clothing from a suspect.
For the last 50 years or so, most of our attention has been on the “are there fibres from the suspect’s garment on the victim’s garment or vice-versa”, the next 50 years or so, should all be about the Leitspuren, that’s where the value lies.
Seats
Another deep sigh, shrug, “its complicated”. Seats are not people. They don’t move about and the intensity and duration of the contact is limited to being determined by one party only - the person sitting on it. So it’s not a surprise that in relation to secondary transfer to the seat12, and from the seat, that the simplicity of the transfer model found on clothing, is replaced with something far more complex. 13
This has led to this warning in the “fibre bible”14 about interpreting mechanisms of transfer of fibres based solely on the numbers of fibres found:
In all interpretations involving primary versus secondary transfer, it is necessary to consider all factors that can have an effect on fibre transfer and persistence. Under certain conditions (not uncommon in forensic science casework), the recovery of 50 (or fewer) fibres may be a strong indication of a primary transfer. There are also circumstances which could result in a secondary transfer of over 50 fibres. Every case must be assessed individually.
Airborne Fibres
“Contactless transfer” as the marketeers would have it. Loved this paper too, mentioned it previously in the deep dive15.
So imagine being so close to someone in an enclosed spacewithout making contact, that you pass through the “cloud” of airborne fibres their clothing has generated just from their garments being in motion whilst worn. At time zero, the numbers of fibres transferring to their clothing is somewhere between 10 and 20 on average, even for airborne fibres generated from garments made from cotton. This is tiny compared to the hundreds and thousands of fibres that transfer at time zero during actual physical contact.
This study tells us nothing about airborne fibre transfer outdoors, passing by people on the street for example. Logic would perhaps dictate that the chances of any airborne fibres depositing on garments would be less than was observed in this study where recipients were in a confined space where you are less than 2m away from the donor, but there is no data at present to assess this.
Another key finding of this study related to the persistence of fibres transferred secondarily. In all instances all fibres transferred secondarily were lost within two hours. In 50% of the experiments, all fibres were lost in 30 minutes post transfer.
Chance Occurrences
Coming back to our target fibre studies and finding small number of fibres by chance. Where are these fibres coming from? Are they the result of a direct contact with a garment made from the same fibres that make up the target garment transferred to that surface at least 6 hours prior to them being recovered, or, have they picked up these fibres via the airborne route or through a secondary transfer mechanism, but within 2 hours of that transfer? Those are the only options.
It’s a proper conundrum to figure that one out.
On the face of it historical direct contacts with other garments that happen to be made of the same fibres as the target look like a good candidate for explaining finding small numbers by chance…but perhaps not when you consider human behaviour. But, how many people, strangers, outside of a small close circle, do we actually make physical contact with in 24 hours16? We might bump into someone maybe, brush past them, share a lift17 with them, be crammed up against them on public transport or a crowded pub or at a football match18 perhaps.
But how many of those would be actually wearing something, at that time, that happens to match the target garment? In the UK with a population of circa 60 million people, each wearing two or three items of clothing on their outermost surfaces that cover most of their surface area, it’s 120-180 million items of clothing per day. If a batch of clothing which is all made of the same stuff is a maximum 100,000 garments -that’s roughly 0.1% of all of the garments worn in the UK in a day. Never mind the fact that there is no way that all of those 100,000 garments would be worn on that day or have an equal chance of being in contact with us as we go about our normal day.
Picking small numbers of fibres up on the other hand would not require the presence of the garment at all in that moment. Instead all it would require is garments worn by to come into contact with any surface which has those fibres on it in that moment. In this instance I like to take the 6 degrees of separation approach and consider 100,000 garments over time having multiple contacts with multiple surfaces leaving fibres on seats and other places that persist for a little longer than the typical persistence curves allow. To me, just as a thought experiment, recent secondary transfer mechanisms look like the likeliest source of these “fortuitous” fibres, but there is currently no scientific data to back up that and Tiernan’s first law of forensic fibre research applies.
Next up in the deep dive, locations, relationships and circumstances, context is everything in forensic fibre examinations.
Nylon, wool, acrylic, viscose (rayon) etc etc
Assuming the way we measure that is robust.
with the caveat of course that each case is unique and would require assessment
A study in relation to the random distribution of four fibre types on clothing (incorporating a review of previous target fibre studies) K. Wiggins, P. Drummond , T. Hicks Champod. Science & Justice Volume 44, Issue 3, July 2004, Pages 141-148
Tiernan’s second law is that you should find another eejit to do the work next time
noun informal• from the Irish: a stupid person; an idiot. "don't stand there like a gormless eejit!"
Solely on the basis that it requires human beings to search for them
Excluding wool as a target fibre and studies conducted in the 1980s
We know that 80% of the fibres are lost within 2-4 hours of a surface being active.
A fibre collective (or group of collectives) recovered from a crime scene or victim where the number; the distribution and as a supplement, the material of the fibres, suggests deposition by the perpetrator.
Setting aside those who have to make contact with people as part of their job of course.
also known as an elevator
Other sports too.