Essay!
Just incase something horrible happens or if anyone is interested in the factors affecting intrepretation and evaluation of fibre evidence....
Textile fibre evidence is often considered especially complex to evaluate in comparison to other forms of trace evidence.
Unlike the genetic loci used in DNA evidence, fibre types are not constant over long periods of time within large populations. DNA evaluation can assign a numerical value to the frequency of a specific loci allele due to the fact that it is known that the allele frequency is stable within a population - it is unlikely to be lost or undergo dramatic transformation within an appreciable timescale. This is not the case for fibres, where new types are constantly being developed, others become obsolete, and the frequency is strongly tied to a tightly specified population and time period
Even other man-made forms of trace, such as paint and glass, do not show such dramatic and rapid alterations. Paint and glass also have the benefit of exhibiting a narrower range of varieties (in comparison with fibres, where dye type, colour, and fibre composition can all be varied), although this does result in a lower level of discrimination.
Fibres have the potential to be highly discriminatory. Not only are many different fibre types available (cotton, wool, silk, polyester, nylon, acrylic to name a few), a variety of types of dye are also available (vat, direct, acid, basic, reactive etc) in an almost limitless range of colours. Dyes can even be combined to produce further end point colours. Other physical characteristics which can be used to describe a fibre are width, cross-sectional shape, lustre, overall form (e.g. whether a kink is present), polymer composition (if synthetic) and distribution of delustrant (if present). In general terms, there is more potential for variation (and thus discrimination) in synthetic fibres compared to natural fibres.
When evaluating the value of textile fibre evidence, it is not sufficient to merely identify the characteristics. Of primary importance is the location from which the fibres were recovered. This is comparable with interpretation of blood patterns - knowing solely where the blood originated from is not enough, and the distribution can give strong indication of the activities which lead to it being present at the scene. The same is true of fibres - for example in a case of sexual assault, the recovery of fibres from the outer garments worn by the assailant on the under garments of the victim, could lend support to the allegation. However, as will become clear, the location of a fibre is not fixed at the point of transfer, and in the course of interpreting results it must be borne in mind that redistribution may have occurred, and that the location fibres are recovered from may not correspond with the location of contact.
Location is of limited use without knowledge of the number of fibres recovered, although this is where things start to get more complicated. Before starting on the significance of numbers of fibres found, it must be realised that low numbers of fibres, no matter how distinctive, should always be treated with caution. The number of fibres transferred is dependant upon the 'sheddability' of the fabric from which the fibre originates. Natural fibres generally shed more easily than synthetic, and those with a 'rough' surface texture more easily than smooth - both these variables are linked to whether the yarn making the fabric is composed of stable or filament fibres, the former being shed more easily. In addition, fabrics which are damaged will also shed fibres well. Moving to consideration of the recipient fabric (which is where fibres will be recovered from), retention of fibres is also dependant on the type of fabric. In broad terms, those which shed well, also retain well. Therefore, the numbers of fibres recovered must be considered in the context of the type of fibre and the likely ease of shedding. If the garment suspected of shedding the recovered fibres is available for examination, the level of shedding can be gauged, and more accurate statements can be made about the type of contact which occurred to result in the fibre transfer observed. Roberson and co-workers (1982) carried out a number of experiments into factors affecting the persistence of fibres, and in addition to those mentioned already, they found that short fibres (i.e. less than 2.5mm in length) are lost more slowly than longer fibres, and also that increased pressure of contact resulted in increased persistence, presumably due to a mechanical interaction resulting in the transferred fibres becoming more firmly embedded in the recipient fabric (proportionally, more of a short fibre is able to become embedded in the fabric compared to one with greater length). A further factor when considering fibre numbers is that a large proportion of fabrics contain more than one fibre type, and often one is shed preferentially (e.g. in a 50:50: cotton polyester mix, more cotton than polyester will be shed - Salter et al., 1987). Other variables affecting persistence of fibres on a garment are wear, washing, and position on a garment. Clearly, fibres are lost though washing, and also through wear, particularly if over-garments (such as jackets) are worn over the item which made contact, but also if fibres are transferred to areas which are subject to friction from other parts of the body (e.g. arms) they will be lost more rapidly.
The numbers of fibres recovered are also often used to indicate whether the fibres are present through primary or secondary transfer. Primary transfer is the term used to describe fibre transfer from one fabric to another, whereas secondary indicates the presence of an intermediate surface e.g. from a blanket, to a jumper worn by a first individual, to a jumper worn by a second individual. This can result in the presence of fibres on an individual from an item with which they have had no direct contact. In comparison with primary transfer, fibres subject to secondary transfer are inevitably found in smaller numbers due to the fact that they are the result of a primary fibre population being redistributed onto a new surface. Work by Pounds and Smalldon (1975) indicated there are different levels of attachment strength for transferred fibres, and that those which are most loosely bound (in general, these are longer fibres) are more likely to take part in secondary transfer. In addition, they noted that the transfer of fibres to a garment with a smooth texture was more likely to result in further onwards fibre transfer than if a rough textured garment were involved. This emphasises the importance of context when examining fibre evidence - it is never enough to merely demonstrate transfer has occurred without considering the garments and circumstances implicated.
Investigations have been carried out into the optimal method of fibre recovery, and the general consensus has been that, in comparison to techniques such as shaking, scraping, vacuuming, and brushing, the use of adhesive tape-lifts are most effective and offer the greatest ease of subsequent examination (Pounds, 1975; Lowrie & Jackson, 1991).
Thus, when fibre samples first arrive in a forensic laboratory for examination, they are in the form of tapelifts. These can contain many hundreds, if not thousands of individual fibres, and so the examiner must make a number of decisions on which fibres to exclude from examination. In most circumstances, indigo/blue cotton fibres are discounted, as these are commonly regarded as a common fibre type (Cantrell et al., 2001). Also discounted are fibres known to be indigenous to the article from which the tape-lift was taken e.g. if the article is a blue woollen jumper, blue woollen fibres will not be examined). It is optimal at this point to have some idea of the types of fibres which are likely to be of interest e.g. matching clothing and other items associated with the suspect.
It is of greatest evidential value if two-way transfer can be demonstrated, that is, fibres from one party being transferred to the other, and vice versa. For example, in a case of alleged physical or sexual assault, it would be expected that the parties involved would be in close contact and that some degree of pressure or force would be present, resulting in conditions inducible to fibre transfer.
The keystone of forensic examination of most physical evidence is comparison. If a suspect is apprehended within hours of an offence, the likelihood of fibres related to the offence being recovered is greatly increased, therefore tapelifts should be taken from their clothing and surroundings (as appropriate) as rapidly as possible. These tapelifts will also provide a background sample which can be used for comparison with other fibres recovered.
When a match is observed between fibres found on a victim and those recovered from a suspect, it would seem reasonable to check that the fibres are not present due to innocent contact with an item which sheds fibres indistinguishable from those attributed to the accused. If intelligence is available regarding the movements of the victim prior to an incident, it would seem logical to test all areas visited for matching fibres. In practice, this is not often feasible - the question becomes one of how far, both spatially and temporally, to go. If no information on movements is available, how does this affect the search for other potential matches? It could be argued that in the quest for the truth surrounding an event, all avenues must be exhausted. In such circumstances, a forensic scientist should compare fibres found on a victim not only with those recovered from a suspect, but also fibres recovered from the victims home, place of work or education (as applicable), friends houses, social areas (such as cinemas, public houses etc), and public transport. Clearly, this would greatly extend the time required to complete an investigation, and it is argued that such an approach would not add enough to an investigation to be justified. A strong argument in favour of only comparing recovered fibres with those from an identified suspect is provided by a study published in 2003 by Houck. He compared fibres recovered from twenty randomly selected items from unrelated cases across America, and was able to distinguish every fibre using the techniques available. The fact that over 2000 fibres were examined and no matches were found seems to suggest that the observation of a match is a significant finding, and therefore it is unnecessary to go to extreme lengths to prove this fact. Although this can only be taken as a guide - the facts of the case must always dictate the specific approach taken.
However, the most crucial factor to bear in mind during fibre evidence evaluation is context. All findings must be considered in conjunction with all other available information.
Also, a lack of fibre evidence does not equate to a lack of contact. There are innumerable factors which lead to rapid fibre loss. It is the job of the forensic scientist to take into account all potential possibilities which could explain the evidence observed, and make a decision on which one they feel is most likely to have occurred.
In the examination of fibres, moreso than with many other areas of trace evidence, the forensic scientist must be prepared to think outside the box - to imagine every possibly scenario which could result in the observed findings and justify any choice of which is most likely, if applicable.
Unlike DNA evidence, it is not yet possible to put a number on the value of fibre evidence. Attempts to create databases have stumbled, largely due to the staggering variety of fibres present in the environment, and the fact that fibre types present are dictated by the season and the current fashion. If a database of fibre frequencies were to be created, it would need to account for changes in frequency throughout the year, and in different geographical regions due to climate and culture. I feel it is therefore the job of the forensic scientist in court, when asked for a fibre 'match ratio', to explain why this is not possibly and convey the fact that fibre evidence, in context, can be of great evidential value.
Textile fibre evidence is often considered especially complex to evaluate in comparison to other forms of trace evidence.
Unlike the genetic loci used in DNA evidence, fibre types are not constant over long periods of time within large populations. DNA evaluation can assign a numerical value to the frequency of a specific loci allele due to the fact that it is known that the allele frequency is stable within a population - it is unlikely to be lost or undergo dramatic transformation within an appreciable timescale. This is not the case for fibres, where new types are constantly being developed, others become obsolete, and the frequency is strongly tied to a tightly specified population and time period
Even other man-made forms of trace, such as paint and glass, do not show such dramatic and rapid alterations. Paint and glass also have the benefit of exhibiting a narrower range of varieties (in comparison with fibres, where dye type, colour, and fibre composition can all be varied), although this does result in a lower level of discrimination.
Fibres have the potential to be highly discriminatory. Not only are many different fibre types available (cotton, wool, silk, polyester, nylon, acrylic to name a few), a variety of types of dye are also available (vat, direct, acid, basic, reactive etc) in an almost limitless range of colours. Dyes can even be combined to produce further end point colours. Other physical characteristics which can be used to describe a fibre are width, cross-sectional shape, lustre, overall form (e.g. whether a kink is present), polymer composition (if synthetic) and distribution of delustrant (if present). In general terms, there is more potential for variation (and thus discrimination) in synthetic fibres compared to natural fibres.
When evaluating the value of textile fibre evidence, it is not sufficient to merely identify the characteristics. Of primary importance is the location from which the fibres were recovered. This is comparable with interpretation of blood patterns - knowing solely where the blood originated from is not enough, and the distribution can give strong indication of the activities which lead to it being present at the scene. The same is true of fibres - for example in a case of sexual assault, the recovery of fibres from the outer garments worn by the assailant on the under garments of the victim, could lend support to the allegation. However, as will become clear, the location of a fibre is not fixed at the point of transfer, and in the course of interpreting results it must be borne in mind that redistribution may have occurred, and that the location fibres are recovered from may not correspond with the location of contact.
Location is of limited use without knowledge of the number of fibres recovered, although this is where things start to get more complicated. Before starting on the significance of numbers of fibres found, it must be realised that low numbers of fibres, no matter how distinctive, should always be treated with caution. The number of fibres transferred is dependant upon the 'sheddability' of the fabric from which the fibre originates. Natural fibres generally shed more easily than synthetic, and those with a 'rough' surface texture more easily than smooth - both these variables are linked to whether the yarn making the fabric is composed of stable or filament fibres, the former being shed more easily. In addition, fabrics which are damaged will also shed fibres well. Moving to consideration of the recipient fabric (which is where fibres will be recovered from), retention of fibres is also dependant on the type of fabric. In broad terms, those which shed well, also retain well. Therefore, the numbers of fibres recovered must be considered in the context of the type of fibre and the likely ease of shedding. If the garment suspected of shedding the recovered fibres is available for examination, the level of shedding can be gauged, and more accurate statements can be made about the type of contact which occurred to result in the fibre transfer observed. Roberson and co-workers (1982) carried out a number of experiments into factors affecting the persistence of fibres, and in addition to those mentioned already, they found that short fibres (i.e. less than 2.5mm in length) are lost more slowly than longer fibres, and also that increased pressure of contact resulted in increased persistence, presumably due to a mechanical interaction resulting in the transferred fibres becoming more firmly embedded in the recipient fabric (proportionally, more of a short fibre is able to become embedded in the fabric compared to one with greater length). A further factor when considering fibre numbers is that a large proportion of fabrics contain more than one fibre type, and often one is shed preferentially (e.g. in a 50:50: cotton polyester mix, more cotton than polyester will be shed - Salter et al., 1987). Other variables affecting persistence of fibres on a garment are wear, washing, and position on a garment. Clearly, fibres are lost though washing, and also through wear, particularly if over-garments (such as jackets) are worn over the item which made contact, but also if fibres are transferred to areas which are subject to friction from other parts of the body (e.g. arms) they will be lost more rapidly.
The numbers of fibres recovered are also often used to indicate whether the fibres are present through primary or secondary transfer. Primary transfer is the term used to describe fibre transfer from one fabric to another, whereas secondary indicates the presence of an intermediate surface e.g. from a blanket, to a jumper worn by a first individual, to a jumper worn by a second individual. This can result in the presence of fibres on an individual from an item with which they have had no direct contact. In comparison with primary transfer, fibres subject to secondary transfer are inevitably found in smaller numbers due to the fact that they are the result of a primary fibre population being redistributed onto a new surface. Work by Pounds and Smalldon (1975) indicated there are different levels of attachment strength for transferred fibres, and that those which are most loosely bound (in general, these are longer fibres) are more likely to take part in secondary transfer. In addition, they noted that the transfer of fibres to a garment with a smooth texture was more likely to result in further onwards fibre transfer than if a rough textured garment were involved. This emphasises the importance of context when examining fibre evidence - it is never enough to merely demonstrate transfer has occurred without considering the garments and circumstances implicated.
Investigations have been carried out into the optimal method of fibre recovery, and the general consensus has been that, in comparison to techniques such as shaking, scraping, vacuuming, and brushing, the use of adhesive tape-lifts are most effective and offer the greatest ease of subsequent examination (Pounds, 1975; Lowrie & Jackson, 1991).
Thus, when fibre samples first arrive in a forensic laboratory for examination, they are in the form of tapelifts. These can contain many hundreds, if not thousands of individual fibres, and so the examiner must make a number of decisions on which fibres to exclude from examination. In most circumstances, indigo/blue cotton fibres are discounted, as these are commonly regarded as a common fibre type (Cantrell et al., 2001). Also discounted are fibres known to be indigenous to the article from which the tape-lift was taken e.g. if the article is a blue woollen jumper, blue woollen fibres will not be examined). It is optimal at this point to have some idea of the types of fibres which are likely to be of interest e.g. matching clothing and other items associated with the suspect.
It is of greatest evidential value if two-way transfer can be demonstrated, that is, fibres from one party being transferred to the other, and vice versa. For example, in a case of alleged physical or sexual assault, it would be expected that the parties involved would be in close contact and that some degree of pressure or force would be present, resulting in conditions inducible to fibre transfer.
The keystone of forensic examination of most physical evidence is comparison. If a suspect is apprehended within hours of an offence, the likelihood of fibres related to the offence being recovered is greatly increased, therefore tapelifts should be taken from their clothing and surroundings (as appropriate) as rapidly as possible. These tapelifts will also provide a background sample which can be used for comparison with other fibres recovered.
When a match is observed between fibres found on a victim and those recovered from a suspect, it would seem reasonable to check that the fibres are not present due to innocent contact with an item which sheds fibres indistinguishable from those attributed to the accused. If intelligence is available regarding the movements of the victim prior to an incident, it would seem logical to test all areas visited for matching fibres. In practice, this is not often feasible - the question becomes one of how far, both spatially and temporally, to go. If no information on movements is available, how does this affect the search for other potential matches? It could be argued that in the quest for the truth surrounding an event, all avenues must be exhausted. In such circumstances, a forensic scientist should compare fibres found on a victim not only with those recovered from a suspect, but also fibres recovered from the victims home, place of work or education (as applicable), friends houses, social areas (such as cinemas, public houses etc), and public transport. Clearly, this would greatly extend the time required to complete an investigation, and it is argued that such an approach would not add enough to an investigation to be justified. A strong argument in favour of only comparing recovered fibres with those from an identified suspect is provided by a study published in 2003 by Houck. He compared fibres recovered from twenty randomly selected items from unrelated cases across America, and was able to distinguish every fibre using the techniques available. The fact that over 2000 fibres were examined and no matches were found seems to suggest that the observation of a match is a significant finding, and therefore it is unnecessary to go to extreme lengths to prove this fact. Although this can only be taken as a guide - the facts of the case must always dictate the specific approach taken.
However, the most crucial factor to bear in mind during fibre evidence evaluation is context. All findings must be considered in conjunction with all other available information.
Also, a lack of fibre evidence does not equate to a lack of contact. There are innumerable factors which lead to rapid fibre loss. It is the job of the forensic scientist to take into account all potential possibilities which could explain the evidence observed, and make a decision on which one they feel is most likely to have occurred.
In the examination of fibres, moreso than with many other areas of trace evidence, the forensic scientist must be prepared to think outside the box - to imagine every possibly scenario which could result in the observed findings and justify any choice of which is most likely, if applicable.
Unlike DNA evidence, it is not yet possible to put a number on the value of fibre evidence. Attempts to create databases have stumbled, largely due to the staggering variety of fibres present in the environment, and the fact that fibre types present are dictated by the season and the current fashion. If a database of fibre frequencies were to be created, it would need to account for changes in frequency throughout the year, and in different geographical regions due to climate and culture. I feel it is therefore the job of the forensic scientist in court, when asked for a fibre 'match ratio', to explain why this is not possibly and convey the fact that fibre evidence, in context, can be of great evidential value.