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ABSTRACT: Mixed DNA profiles are being encountered more frequently as laboratories analyze increasing amounts of touch evidence. If it is determined that an individual could be a possible contributor to the mixture, it is necessary to perform a statistical analysis to allow an assignment of weight to the evidence. Currently, the combined probability of inclusion (CPI) and the likelihood ratio (LR) are the most commonly used methods to perform the statistical analysis. A third method, random match probability (RMP), is available. This article compares the advantages and disadvantages of the CPI and LR methods to the RMP method. We demonstrate that although the LR method is still considered the most powerful of the binary methods, the RMP and LR methods make similar use of the observed data such as peak height, assumed number of contributors, and known contributors where the CPI calculation tends to waste information and be less informative.
Journal of Forensic Sciences 02/2013; · 1.23 Impact Factor
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ABSTRACT: In this paper we investigate the relationship between heterozygous balance (h) and average peak height (ϕ¯) in a set of Identifiler™ data. The mean of heterozygote balance is unaffected by average peak height but the variance about this mean is much lower at high average peak heights.
Forensic science international. Genetics 08/2012; 6(6):729-34. · 2.42 Impact Factor
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ABSTRACT: An evaluation was carried out to determine the effect on routine forensic calculations when incorporating STRs D12S391 and vWA. These loci are co-located on the same arm of chromosome 12. It has been suggested that allelic association could result in over-estimates of strength-of-evidence calculations. In the first place, we argue that is very unlikely that genotypes collected from typical cosmopolitan forensic databases can provide meaningful information about effects attributable to physical linkage. Since admixture is the most likely cause of allelic association in modern populations we specifically evaluate this effect. We use computer simulation as the preferred approach to generate populations with disequilibrium and observe the effect on match probability. Although we have specifically evaluated the linkage between D12S391 and vWA, the methods described in this paper can be extended and generalized to evaluate linkage effects between any pair of loci where the recombination rate is known. Many jurisdictions apply a subpopulation correction following the standard method of Balding and Nichols. Such corrections would appear to be more than adequate to compensate for any increase in match probability that we were able to create by this admixture. Linkage is likely to have an appreciable effect on relatedness calculations in short pedigrees in some but not all instances. We examined those circumstances where an effect is likely and give formulae for some common situations. The complexity of these calculations is a cause for concern in some laboratories. We discuss possible strategies that might be employed and plausible effects.
Forensic science international. Genetics 12/2011; 6(4):477-86. · 2.42 Impact Factor
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ABSTRACT: The interpretation of lineage markers is usually carried out as a count in a database. The count is a factual statement and does not take into account subpopulation effects that may be acting on the data. Subpopulation effects are usually taken into consideration for autosomal DNA genotype interpretation by the incorporation of a correction, θ. The question has arisen as to whether lineage markers should also have such a correction. This paper discusses if and how subpopulation effects could be considered.
Forensic science international. Genetics 09/2011; 6(3):393-7. · 2.42 Impact Factor
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ABSTRACT: Composite profiles are created by combining DNA profiling information from replicate profiles derived from the same DNA extract. In this paper we have shown that, provided the probability of drop-in is low or nil, the creation of composite profiles is an acceptable approximation to a Bayesian approach, as long as simple samples are analysed.
Forensic science international. Genetics 08/2011; 6(3):317-21. · 2.42 Impact Factor
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ABSTRACT: This is a discussion of a number of issues that arise from the recent judgment in R v T [1]. Although the judgment concerned with footwear evidence, more general remarks have implications for all disciplines within forensic science. Our concern is that the judgment will be interpreted as being in opposition to the principles of logical interpretation of evidence. We reiterate those principles and then discuss several extracts from the judgment that may be potentially harmful to the future of forensic science. A position statement with regard to evidence evaluation, signed by many forensic scientists, statisticians and lawyers, has appeared in this journal [2] and the present paper expands on the points made in that statement.
Science & Justice 06/2011; 51(2):43-9. · 1.60 Impact Factor
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ABSTRACT: The occurrence of mixed DNA profiles in forensic samples is not uncommon. Interpretation of these profiles however can be challenging. In this paper we compare three different models for interpreting mixed DNA profiles prior to the calculation of a statistical weight. Two of these models take into account the peak height of the alleles. The third method uses an unconstrained combinatorial approach. We compare the statistical weights calculated after applying the three different models to one low level two-person mixed DNA profile derived from a crime sample and provide tables of equations that can be applied to many different scenarios including single source and two and three person mixed DNA profiles.
Forensic science international. Genetics 05/2011; 6(2):191-7. · 2.42 Impact Factor
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ABSTRACT: Understanding the behaviour of mixed DNA profiles is of paramount importance in forensic DNA analysis. Key parameters are those of heterozygote balance and mixture proportion and its variability. These parameters have been previously explored as a function of the average peak height of the active alleles in single source and mixed samples derived from pristine DNA. Here we report a comparison of this data with data obtained from casework samples. This allows an assessment of the difference in the distribution of heterozygote balance between mixed and single source stains and between casework mixtures and synthetic mixtures constructed from pristine DNA.
Forensic science international. Genetics 05/2011; 6(2):180-4. · 2.42 Impact Factor
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ABSTRACT: Stutter is an artefact seen when amplifying short tandem repeats and typically occurs at one repeat unit shorter in length than the parent allele. In forensic analysis, stutter complicates the analysis of DNA profiles from multiple contributors, known as mixed profiles, a common profile type. Consequently it is important to both understand and predict stutter behaviour in order to improve our understanding of the resolution and interpretation of these profiles. Whilst stutter is well recognised and documented, little information is available that identifies and quantifies what influences the formation of stutter. In this work we use a novel approach to examine this. We have used synthetic oligonucleotides comprising multiple repeat units to test; the influence of repeat number, the influence of repeat sequence and the impact of interruptions to the repeat sequence length. Using multiple replicates allows detailed statistical analysis. We have confirmed a linear relationship between stutter ratio and repeat number. We have shown that increased A-T content increases stutter ratio and that interruptions in repeating sequences decreased stutter ratios to levels similar to the longest uninterrupted repeat stretch. We also found that there was no relationship between stutter ratio and repeat number for a repeat unit with an A-T content of 1/4 and that half of the interrupted repeat sequences stuttered significantly less than their longest uninterrupted repeat stretches. We have applied the knowledge gained to examine specific features of the loci present in the AmpFlSTR(®) SGM Plus(®) multiplex kit used in our laboratory.
Forensic science international. Genetics 03/2011; 6(1):58-63. · 2.42 Impact Factor
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ABSTRACT: The interpretation of mixed DNA profiles presents additional challenges for the forensic scientist. There has been a broad based call for transparency in the process of interpretation of all evidence including mixed DNA profiles. This interpretation is greatly facilitated by a sound understanding of the variability in peak heights for the two peaks of a heterozygote, in the sizes of stutter peaks and in the variability in peak heights across loci. This study examines single source and mixed DNA profiles to assess this variability. The relative variability in peak height between the two peaks of a heterozygote and in the peak heights across loci becomes greater as the peaks themselves become smaller. This is consistent with findings from other multiplexes. This variability appears larger in the MiniFiler™ system at 30 cycles than, for example, in the Identifiler™ system at 28 cycles and this difference is largely explained by the two extra cycles of amplification. Stutter peaks appear no larger in the MiniFiler™ system at 30 cycles than in the Identifiler™ system at 28 cycles.
Forensic science international. Genetics 10/2010; 5(5):381-5. · 2.42 Impact Factor
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ABSTRACT: Recent discussions on a forensic discussion group highlighted the prevalence of a practice in the application of inclusion probabilities when dropout is possible that is of significant concern. In such cases, there appears to be an unpublished practice of calculation of an inclusion probability only for those loci at which the profile of interest (hereafter the suspect) is fully included among the alleles present in the crime scene sample and to omit those loci at which the suspect has alleles that are not fully represented among the alleles in the mixture. The danger is that this approach may produce apparently strong evidence against a surprisingly large fraction of noncontributors. In this paper, the risk associated with the approach of ignoring loci with discordant alleles is assessed by simulation.
Journal of Forensic Sciences 09/2010; 55(5):1171-3. · 1.23 Impact Factor
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Forensic science international. Genetics 05/2010; · 2.42 Impact Factor
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ABSTRACT: The interpretation of mixed DNA profiles is often undertaken by determining to what extent possible genotype combinations may be eliminated from consideration. This requires an understanding of the variability in height or area for the two alleles of a heterozygote (heterozygote balance), and in the variability of mixture proportion or mixture ratio across loci. We present here empirical data to help assess the magnitude of this variability for Identifiler profiles. We find that heterozygote balance is affected by peak height as expected and that at heights above 267RFU the two peaks of a heterozygote demonstrates a peak height ratio between 0.6 and 1.66. For mixtures we introduce a concept, APH, the average peak heights of the active alleles. Above an APH of 110RFU mixture proportion demonstrated variation no more than 0.2 from the average across loci. Mixture ratio variability was affected by both the mixture ratio, M(r) per se and by the APH. The function (2APH/(1+M(r))) appears to be a reasonable predictor of the variability and approximates the expected peak height of the minor alleles. At (2APH/(1+M(r)))> or =300RFU the mixture ratio at a locus is expected to be within a factor of 2 of the average across loci.
Forensic science international. Genetics 02/2010; 4(2):111-4. · 2.42 Impact Factor
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Journal of Forensic Sciences 01/2010; 55(1):265-8; author reply 269-72. · 1.23 Impact Factor
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ABSTRACT: We discuss the interpretation of DNA profiles obtained from low template DNA samples. The most important challenge to interpretation in this setting arises when either or both of "drop-out" and "drop-in" create discordances between the crime scene DNA profile and the DNA profile expected under the prosecution allegation. Stutter and unbalanced peak heights are also problematic, in addition to the effects of masking from the profile of a known contributor. We outline a framework for assessing such evidence, based on likelihood ratios that involve drop-out and drop-in probabilities, and apply it to two casework examples. Our framework extends previous work, including new approaches to modelling homozygote drop-out and uncertainty in allele calls for stutter, masking and near-threshold peaks. We show that some current approaches to interpretation, such as ignoring a discrepant locus or reporting a "Random Man Not Excluded" (RMNE) probability, can be systematically unfair to defendants, sometimes extremely so. We also show that the LR can depend strongly on the assumed value for the drop-out probability, and there is typically no approximation that is useful for all values. We illustrate that ignoring the possibility of drop-in is usually unfair to defendants, and argue that under circumstances in which the prosecution relies on drop-out, it may be unsatisfactory to ignore any possibility of drop-in.
Forensic science international. Genetics 12/2009; 4(1):1-10. · 2.42 Impact Factor
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ABSTRACT: The characteristics of STR profiles produced from approximately 1 ng starting template using the AMPFlSTR SGM Plus multiplex and 28 PCR cycles, are well documented. However, the analysis of samples perceived as low in starting template (less than 100 pg), and referred to as low template DNA (LTDNA), can require a test of higher sensitivity in order to achieve successful results. One way of increasing this sensitivity is to increase the number of PCR amplification cycles from 28 to 34. This type of analysis has become known as low copy number, or LCN, DNA profiling. Amplification of LTDNA under LCN conditions can result in increased incidents of profile characteristics such as allelic 'drop-in' and allelic 'drop-out'. Adopting a testing regime which includes duplicate analysis, and maintaining a laboratory environment of stringent and monitored cleanliness, enables the scientist to identify and control these phenomena for a reliable interpretation of the DNA profiling results. A recent court ruling has questioned the reliability of LCN analysis and commented on the paucity of publications surrounding the validation of the technique. We present data for the LCN validation undertaken in our laboratory, and describe the guidelines and working practices we have developed for the analysis and interpretation of profiles generated after LCN profiling. This study augments the published record relating to LCN validation and should act as a useful guide for other laboratories who are considering implementing LCN profiling.
Forensic science international. Genetics 12/2009; 4(5):305-10. · 2.42 Impact Factor
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ABSTRACT: In this paper we critically examine the causes of the underlying confusion that relates to the issue of low-template (LT) DNA profile interpretation. Firstly, there is much difficulty in attempting to distinguish between LT-DNA vs. conventional DNA because there is no discrete 'cut-off' point that can be reasonably defined or evaluated. LT-DNA is loosely characterised by drop-out (where alleles may be missing) and drop-in (where additional alleles may be present). We have previously described probabilistic methods that can be used to incorporate these phenomena using likelihood ratio (LR) principles. This is preferred to the random man not excluded (RMNE) method, because we cannot identify a coherent way forward within the restrictions provided by this framework. Most LT-DNA profiles are interpreted using a 'consensus' profile method, we called this the 'biological model', where only those alleles that are duplicated in consecutive tests are reported. We recognise that there is an increased need for probabilistic models to take precedence over the biological model. These models are required for all kinds of DNA profiles, not just those that are believed to be low-template. We also recognise that there is a need for education and training if the methods we recommend are to be widely introduced.
Forensic science international. Genetics 10/2009; 4(4):221-7. · 2.42 Impact Factor
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John Buckleton
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ABSTRACT: A recent ruling in the Crown Court of Northern Ireland, R v. Hoey, [R v Sean Hoey. 2007, Crown Court of Northern Ireland] has raised questions about the validity of one variant of DNA analysis, often termed LCN. The ruling and subsequent discussion also raises questions about what constitutes validation of a technique. This paper examines what can be achieved in a laboratory based validation study against the Daubert standard and against guidance given in the UK. There is a significant discrepancy between what can be achieved and the Daubert standard but much less of a discrepancy against the UK guidance. Much of the difference relates to differences in word usage, definitional difficulties, and a lack of mutual understanding and communication between the judiciary and forensic scientists. This highlights a gap that needs attention.
Forensic science international. Genetics 10/2009; 3(4):255-60. · 2.42 Impact Factor
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ABSTRACT: Guidance exists on how to count matches between samples in a crime sample database but we are unable to locate a definition of how to estimate a match rate. We propose a method that does not proceed from the match counting definition but which has a strong logic.
Forensic science international. Genetics 07/2009; 3(3):200-1. · 2.42 Impact Factor
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ABSTRACT: The result of empirical testing of forensic DNA match probabilities for Croatia is reported. It is concluded that if consideration is given to relatedness and subpopulation effects the model of Balding and Nichols appears to give very good predictions.
Forensic science international. Genetics 01/2009; 3(1):50-3. · 2.42 Impact Factor
