Anders Nordgaard

Improved forensic DNA analysis through the use of alternative DNA polymerases and statistical modeling of DNA profiles

DNA evidence, linking perpetrators to crime scenes, is central to many legal proceedings. However, DNA samples from crime scenes often contain PCR-inhibitory substances, which may generate blank or incomplete DNA profiles. Extensive DNA purification can be required to rid the sample of these inhibitors, although these procedures increase the risk of DNA loss. Most forensic laboratories use commercial DNA amplification kits (e.g., AmpFlSTR SGM Plus) with the DNA polymerase AmpliTaq Gold as the gold standard. Here, we show that alternative DNA polymerase-buffer systems can improve the quality of forensic DNA analysis and efficiently circumvent PCR inhibition in crime scene samples, without additional sample preparation. DNA profiles from 20 of 32 totally or partially inhibited crime scene saliva samples were significantly improved using Bio-X-Act Short, ExTaq Hot Start, or PicoMaxx High Fidelity instead of AmpliTaq Gold. A statistical model for unbiased quality control of forensic DNA profiles was developed to quantify the results. Our study demonstrates the importance of adjusting the chemistry of the PCR to enhance forensic DNA analysis and diagnostic PCR, providing an alternative to laborious sample preparation protocols.

Resampling Stochastic Processes using a Bootstrap Approach

A bootstrap method for weakly stationary Gaussian sequences is presented. The resampling is done among the coefficients of the random Fourier representation of a sample from the sequence. The independent increments for the spectral representation of a sequence justifies the choice of the method and the resampling provides new bootstrap realizations. Validity of the method is checked on a typical spectral estimator and a correlation estimator using computer simulation of large samples. The results are satisfactory and point towards asymptotic validity of the method.

A ranking index for quality assessment of forensic DNA profiles

BackgroundAssessment of DNA profile quality is vital in forensic DNA analysis, both in order to determine the evidentiary value of DNA results and to compare the performance of different DNA analysis protocols. Generally the quality assessment is performed through manual examination of the DNA profiles based on empirical knowledge, or by comparing the intensities (allelic peak heights) of the capillary electrophoresis electropherograms.ResultsWe recently developed a ranking index for unbiased and quantitative quality assessment of forensic DNA profiles, the forensic DNA profile index (FI) (Hedman et al. Improved forensic DNA analysis through the use of alternative DNA polymerases and statistical modeling of DNA profiles, Biotechniques 47 (2009) 951-958). FI uses electropherogram data to combine the intensities of the allelic peaks with the balances within and between loci, using Principal Components Analysis. Here we present the construction of FI. We explain the mathematical and statistical methodologies used and present details about the applied data reduction method. Thereby we show how to adapt the ranking index for any Short Tandem Repeat-based forensic DNA typing system through validation against a manual grading scale and calibration against a specific set of DNA profiles.ConclusionsThe developed tool provides unbiased quality assessment of forensic DNA profiles. It can be applied for any DNA profiling system based on Short Tandem Repeat markers. Apart from crime related DNA analysis, FI can therefore be used as a quality tool in paternal or familial testing as well as in disaster victim identification.

Assessment of Approximate Likelihood Ratios from Continuous Distributions: A Case Study of Digital Camera Identification*

Abstract:  A reported likelihood ratio for the value of evidence is very often a point estimate based on various types of reference data. When presented in court, such frequentist likelihood ratio gets a higher scientific value if it is accompanied by an error bound. This becomes particularly important when the magnitude of the likelihood ratio is modest and thus is giving less support for the forwarded proposition. Here, we investigate methods for error bound estimation for the specific case of digital camera identification. The underlying probability distributions are continuous and previously proposed models for those are used, but the derived methodology is otherwise general. Both asymptotic and resampling distributions are applied in combination with different types of point estimators. The results show that resampling is preferable for assessment based on asymptotic distributions. Further, assessment of parametric estimators is superior to evaluation of kernel estimators when background data are limited.

Comments on “The database search problem” with respect to a recent publication in Forensic Science International

Comments on ‘‘The database search problem’’ with respect to a recent publication in Forensic Science International : Letter to the Editor

Chemometrics comes to court: evidence evaluation of chem-bio threat agent attacks

Forensic statistics is a well‐established scientific field whose purpose is to statistically analyze evidence in order to support legal decisions. It traditionally relies on methods that assume small numbers of independent variables and multiple samples. Unfortunately, such methods are less applicable when dealing with highly correlated multivariate data sets such as those generated by emerging high throughput analytical technologies. Chemometrics is a field that has a wealth of methods for the analysis of such complex data sets, so it would be desirable to combine the two fields in order to identify best practices for forensic statistics in the future. This paper provides a brief introduction to forensic statistics and describes how chemometrics could be integrated with its established methods to improve the evaluation of evidence in court.

Forensic characterization of mid-range petroleum distillates using light biomarkers

ABSTRACT Due to oil refining, commonly used higher boiling biomarkers for oil-source correlation are absent from mid-range petroleum distillates, while lighter biomarkers are concentrated in such products. This study evaluated 63 diagnostic ratios of light biomarkers such as bicyclic sesquiterpanes, diamondoids, and lighter aromatic compounds using 70 diesel oil samples obtained from three Swedish refineries and local gas stations, mostly over a six-month period in 2015. On the basis of their diagnostic power and partial correlation coefficients, a set of 24 ratios is suggested for oil-source correlation of lighter products. The frequency of false positives for this set was determined to be approximately 0.1%. It should be emphasized that in the event of an oil spill, diesel oils are rapidly influenced by weathering and many of the ratios will be affected.

The Roles of Participants’ Differing Background Information in the Evaluation of Evidence

Sir, There is a continuing debate about the role of background information in the evaluation ofevidence. There is concern that different participants in the legal process have different background information and that this causes difficulty in the evaluation of evidence. We argue that although forensic scientists and fact finders have different information, no difficulty is caused in the evaluation process. This argument is particularly apposite given an argument in a recent article by Lund and Iyer (1). They argue that ‘[b]ecause the likelihood ratio is subjective and personal, we find that the proposed framework in which a forensic expert provides a likelihood ratio for others to use in Bayes’ equation is unsupported by Bayesian decision theory, which applies only to personal decision making and not to the transfer of information from an expert to a separate decision maker, such as a juror’. It is argued here that this finding is not valid. The likelihood ratio provided by the forensic scientist is a valid method for the transfer of information from an expert to a separate decision maker.The odds form of Bayes’ theorem for the evaluation of evidence is often formulated as

The optimal number of offspring per gamete donor

Our aim was to create a mathematical basis to calculate the risks for unintended matings of consanguineous half‐siblings from a donor in a society with approximately 10 million inhabitants. The Curie–Cohen model for calculation of the risk for consanguineous mating was used. When the number of offspring per donor is limited to 10, then the model gives a yearly risk for consanguineous matings below 1%. Thus 10 offspring gives a risk for consanguineous matings of 0.9% per year, or approximately once in every 100 years. The risk increases exponentially: with 15 offspring it exceeds 2% and with 25 it reaches up above 5%.

Commentary: Likelihood Ratio as Weight of Forensic Evidence: A Closer Look

A recent article (Lund and Iyer, 2017) provides, in the words of its title, a closer look at the likelihood ratio as the weight of forensic evidence. This note comments critically on two aspects of the article.A commentary on Likelihood Ratio as Weight of Forensic Evidence: A Closer Look : by Lund, S. P., and Iyer, H. (2017). J. Res. Natl. Inst. Stand. Technol. 122:27