Finding the way forward for forensic science in the US-A commentary on the PCAST report.

A recent report by the US President's Council of Advisors on Science and Technology (PCAST), (2016) has made a number of recommendations for the future development of forensic science. Whereas we all agree that there is much need for change, we find that the PCAST report recommendations are founded on serious misunderstandings. We explain the traditional forensic paradigms of match and identification and the more recent foundation of the logical approach to evidence evaluation. This forms the groundwork for exposing many sources of confusion in the PCAST report. We explain how the notion of treating the scientist as a black box and the assignment of evidential weight through error rates is overly restrictive and misconceived. Our own view sees inferential logic, the development of calibrated knowledge and understanding of scientists as the core of the advance of the profession.

The interpretation of lineage markers in forensic DNA testing.

Mitochondrial DNA (mtDNA) and the non-recombining portion of the Y-chromosome are inherited matrilinealy and patrilinealy, respectively, and without recombination. Collectively they are termed 'lineage markers'. Lineage markers may be used in forensic testing of an item, such as a hair from a crime scene, against a hypothesised source, or in relationship testing. An estimate of the evidential weight of a match is usually provided by a count of the occurrence in some database of the mtDNA or Y-STR haplotype under consideration. When the factual statement of a count in the database is applied to a case, issues of relevance of the database and sampling uncertainty may arise. In this paper, we re-examine the issues of sampling uncertainty, the relevance of the database, and the combination of autosomal and lineage marker evidence. We also review the recent developments by C.H. Brenner.

An investigation into the relationship between edge counts and the variability of the refractive index of glass. Part I: Edge morphology.

The effect of glass fragment morphology on the variability of refractive index values in five different glass objects was investigated. Data are presented that suggest that the variability of refractive index values is increased when fragment edge morphology becomes unsuitable for phase contrast microscopy.

An extended likelihood ratio framework for interpreting evidence.

This paper reviews some current methods, the likelihood ratio-based approach and the full Bayesian approach for the interpretation of evidence and discusses previously identified shortcomings in them. It suggests an approach based on a compromise--based on an extended likelihood ratio--that may combine the merits of logic without overstepping acceptable bounds for the forensic scientist in the presentation of evidence. The approach is exposed formally and takes advantage of inferential networks called Bayesian networks.

DNA commission of the International Society of Forensic Genetics: Recommendations on the interpretation of mixtures.

The DNA commission of the International Society of Forensic Genetics (ISFG) was convened at the 21st congress of the International Society for Forensic Genetics held between 13 and 17 September in the Azores, Portugal. The purpose of the group was to agree on guidelines to encourage best practice that can be universally applied to assist with mixture interpretation. In addition the commission was tasked to provide guidance on low copy number (LCN) reporting. Our discussions have highlighted a significant need for continuing education and research into this area. We have attempted to present a consensus from experts but to be practical we do not claim to have conveyed a clear vision in every respect in this difficult subject. For this reason, we propose to allow a period of time for feedback and reflection by the scientific community. Then the DNA commission will meet again to consider further recommendations.

A study of the performance and utility of annealing in forensic glass analysis.

The use of annealing in forensic glass casework is reviewed. New data is presented that suggests a continuous approach to the interpretation of such data may be superior to the classification approach. Data are presented supporting the previously observed relationship between change in refractive index (RI) and the thickness of the glass pane. An examination of the possible assumption of independence between RI and change in RI is undertaken.

The two trace transfer problem re-examined.

The classical two trace transfer problem initially discussed by Evett is reconsidered [Evett IW. On Meaningful Questions: A Two-Trace Transfer Problem. Journal of the Forensic Science Society 1987; 27: 375-381]. In this problem we consider the situation where there are two stains of differing types at a crime scene. Evett considered the situation where both are known to be relevant to the offence. It was further assumed that there were exactly two offenders. Reanalysis of this problem suggests that the key consideration is the conditioning on there being exactly two offenders, not that there were two stains. The treatment here leads easily to a generalisation in which stains have differing relevance and there are k offenders.

Effects of population structure and admixture on exact tests for association between Loci.

The probability of multilocus genotype counts conditional on allelic counts and on allelic independence provides a test statistic for independence within and between loci. As the number of loci increases and each sampled genotype becomes unique, the conditional probability becomes a function of total heterozygosity. In that case, it does not address between-locus dependence directly but only indirectly through detection of the Wahlund effect. Moreover, the test will reject the hypothesis of allelic independence only for small values of heterozygosity. Low heterozygosity is expected for population subdivision but not for population admixture. The test may therefore be inappropriate for admixed populations. If individuals with parents in two different populations are always considered to belong to one of the populations, then heterozygosity is increased in that population and the exact test should not be used for sparse data sets from that population. If such a case is suspected, then alternative testing strategies are suggested.

Allele frequencies for the four major sub-populations of New Zealand at three STR loci--HUMTH01, HUMTPOX and CSF1PO.

Allele frequencies for the three STR loci included in the GenePrint CTT multiplex system (HUMTH01, HUMTPOX, HUMCSF1PO) have been determined for the four major sub-populations of New Zealand.

Assessing uncertainty in DNA evidence caused by sampling effects.

Sampling error estimation in forensic DNA testimony was discussed. Is an estimate necessary and how should it be made? The authors find that all modern methods have areas of strength and weakness. The assessment of which is the 'best' is subjective and depends on the performance of the method, the type of problem (criminal work or paternity), the database size and availability of computing software and support. The authors preferred the highest posterior density approach for performance, however the other methods all have areas where their performance is adequate. For single-contributor stains normal approximation methods are suitable, also the bootstrap and the highest posterior density method. For multiple-contributor stains or other complex situations the match probability expressions become quite complex and it may not be possible to derive the necessary variance expressions. The highest posterior density or the bootstrap provide a better general method, with non-zero theta. The size-bias correction and the factor of 10 approaches may be considered acceptable by many forensic scientists as long as their limitations are understood.

How many samples from a drug seizure need to be analyzed?

Recently Aitken introduced an outstanding advance in the approach to decision making regarding drugs sampling. Unfortunately this approach has not, as yet, been widely implemented despite being based on a solid mathematical foundation. In this paper we advocate a Bayesian approach along the lines of that outlined by Aitken but designed to be both easily understood with less mathematical sophistication and implementable using standard EXCEL software. The emphasis is placed on encouraging the application of this methodology to routine case work by explaining the statistics involved. Minor differences exist between this approach and that of Aitken in both the modeling of the prior probability and in dealing with the discrete nature of the samples. These differences in no way detract from the sound mathematical foundation of the approach.

The fallacy of independence testing and the use of the product rule.

Use of the product rule which implies the assumption of within and between locus independence, is still common, particularly in the United States of America. Whilst it may be considered by some to be an acceptable approximation it is not logical to suggest that independence testing somehow "validates" its use. This paper discusses the nature of this fallacy.

Glass on clothing and shoes of members of the general population and people suspected of breaking crimes.

The outer clothing and footwear of 122 people attending a university gymnasium and a private gymnasium were searched for fragments of glass. Both the surfaces and the pockets of the clothing and the uppers and soles of the footwear were searched. New Zealand forensic glass cases have been reviewed to determine the amount of non-matching glass present on the clothing of people who are suspected of breaking crimes. Data from 114 suspects who had no matching glass on their clothing and shoes were accumulated. Statistical modelling techniques have been applied to the data collected.

Assessing transfer probabilities in a Bayesian interpretation of forensic glass evidence.

When someone breaks glass a number of tiny fragments may be transferred to that person. If the glass is broken in the commission of a crime then these fragments may be used as evidence. A Bayesian interpretation of this evidence relies on the forensic scientist's ability to assess the probability of transfer. This paper examines the problem of assessing this probability and suggests some solutions.

Setting bounds for the likelihood ratio when multiple hypotheses are postulated.

The interpretation of mixtures of DNA in the forensic context presents particular challenges. The only logical means available for dealing with them is through Bayesian inference, which leads to the formulation, in most cases, of a likelihood ratio which weighs the evidence in favour of two competing hypotheses. However, situations can arise in which additional hypotheses are proposed and the authors discuss one such situation--that where the number of contributors to the mixture is in dispute. A way of dealing with the problem is presented.

Applications and extensions of subpopulation theory: a caseworkers guide.

This paper extends the calculation of conditional probabilities from those given by Balding and Nichols to casework situations where a series of possible DNA types are possible. Such situations may occur when a sample is identified containing a mixture of DNA from two or more people or where extra information can be determined about the subpopulation under consideration by analysis of additional samples. Using this approach, the error in the estimated likelihood ratios is expected to reduce as the number of additional individuals typed from the subpopulation increases.

New Zealand population data at five VNTR loci: validation as databases for forensic identity testing.

Databases were developed for three New Zealand ethnic groups (Caucasian, Maori and Polynesian), at five VNTR loci (D1S7, D2S44, D4S139, D5S110 and D12S11), and validated for interpretation of forensic identity tests. A +/-2.8% sliding window was used to define the alleles at each locus and allelic frequency distributions were obtained for each locus. The conservative nature of the sliding window approach for forensic casework was demonstrated. Tests for independence of alleles within and between loci showed good agreement with the expectation of independence. Although Polynesians are known to have reduced genetic diversity at some VNTR loci, this was found not to be a concern for the present methodology. Procedures for the analysis and reporting of DNA profile results used by New Zealand forensic scientists are therefore appropriate.

Statistical analysis of a large file of data from STR profiles of British Caucasians to support forensic casework.

Data from nearly 2500 British Caucasians, profiled using an STR quadruplex, have been analysed. The data came from several laboratories and represent samples from different geographical distributions. Analysis of the combined files shows that previous reports of failed independence tests were the results of sampling effects. A further convincing proof is given of the robustness of the statistical methods used to estimate evidential value in casework. Comparisons between different samples show that regional effects between Scotland and the South of England have no importance from the forensic viewpoint.

Calculating the frequency of occurrence of a blood type for a 'random man'.

A scheme is offered for the calculation of blood group frequencies for mixed racial populations. The issue of which population to model is discussed. Consideration is also given to the racial composition of the desired survey and to the rule of mutual independence. The scheme can in many cases be applied to existing data.