Advanced Topics in Forensic DNA Typing: Interpretation - John M. Butler

Advanced Topics in Forensic DNA Typing: Interpretation (eBook)

John M. Butler (Autor)

eBook Download: PDF | EPUB

2014 | 1. Auflage
608 Seiten
Elsevier Science (Verlag)
978-0-12-405854-5 (ISBN)

The first edition of Forensic DNA Typing, published in 2001, by John Butler quickly established itself as the gold-standard reference for the field. Over the next ten years, the vast amount of new information uncovered has resulted in this new volume, Advanced Topics in Forensic DNA Typing: Interpretation. This book builds upon the previous two editions of Forensic DNA Typing books, but with a focus on mixture interpretation and statistical analysis and is a companion to the bestselling Advanced Topics in Forensic DNA Typing: Methodology, published in September 2011.

Provides forensic DNA analysts coverage of the crucial topic of DNA mixture interpretation and statistical analysis of DNA evidence
Worked mixture examples illustrate the impact of different statistical approaches for reporting results
Includes allele frequencies for 24 commonly used autosomal STR loci, the revised Quality Assurance Standards which went into effect September 2011

Advanced Topics in Forensic DNA Typing: Interpretation builds upon the previous two editions of John Butler's internationally acclaimed Forensic DNA Typing textbook with forensic DNA analysts as its primary audience. Intended as a third-edition companion to the Fundamentals of Forensic DNA Typing volume published in 2010 and Advanced Topics in Forensic DNA Typing: Methodology published in 2012, this book contains 16 chapters with 4 appendices providing up-to-date coverage of essential topics in this important field. Over 80 % of the content of this book is new compared to previous editions. Provides forensic DNA analysts coverage of the crucial topic of DNA mixture interpretation and statistical analysis of DNA evidence Worked mixture examples illustrate the impact of different statistical approaches for reporting results Includes allele frequencies for 24 commonly used autosomal STR loci, the revised Quality Assurance Standards which went into effect September 2011

Chapter 1

Data Interpretation Overview

Abstract

If the companion volume entitled Advanced Topics in Forensic DNA Typing: Methodology begins with an evidentiary biological sample from a crime scene, then this book begins with a computer file. This computer file contains data points corresponding to time and fluorescence intensity at various wavelengths of light that represent the digital signature of a DNA profile. When these data points are plotted with time on the x-axis and fluorescence intensity on the y-axis, an electropherogram is created. This electropherogram, sometimes referred to as an EPG or e-gram, is then evaluated using STR genotyping software to produce a final results table representing the biological sample’s DNA profile.

Keywords

Data Interpretation; DNA Profile; Standard Operating Procedure (SOP); STR Typing; Validation

“We see the world, not as it is, but as we are – or, as we are conditioned to see it.”

Stephen R. Covey (The 7 Habits of Highly Effective People, p. 28)

Purpose of This Book

This book is primarily intended for DNA analysts or those trying to understand what a DNA analyst does in his or her review of forensic DNA data that was obtained by polymerase chain reaction (PCR) amplification and short tandem repeat (STR) typing via capillary electrophoresis (CE). A DNA analyst, according to the FBI Quality Assurance Standards (QAS) that govern U.S. laboratories, is an individual who “conducts and/or directs the analysis of forensic samples, interprets data and reaches conclusions” (QAS 2011, definitions). Many laboratories employ technicians to perform the analytical techniques required to obtain a DNA profile from a biological sample – typically under the supervision of a trained and qualified analyst. However, as noted by the QAS, “technicians do not interpret data, reach conclusions on typing results, or prepare final reports” (QAS 2011, definitions). Thus, there is an expectation that DNA analyst training involves developing an understanding and mastery of data interpretation as well as report writing and statistical analysis used in reaching conclusions on typing results.

The general steps and workflow involved in forensic DNA typing are illustrated in Figure 1.1. The companion volume to this book entitled Advanced Topics in Forensic DNA Typing: Methodology (Butler 2012) covered many aspects of gathering the data used in DNA testing. Picking up where that book left off, the purpose of this book, Advanced Topics in Forensic DNA Typing: Interpretation, is to help readers understand data obtained from the STR typing process, with a focus on interpreting and reporting results. Data interpretation is covered in Chapters 1 through 8 where we address the question, “What are the data obtained from a set of samples?” Statistical interpretation is reviewed in Chapters 9 through 15 to help discuss, “How significant are the data?” Chapter 16 focuses on drawing conclusions and report writing to assess, “What do the data mean when comparisons are made between evidentiary and reference sample results?”

FIGURE 1.1 Steps involved in the overall process of forensic DNA typing. This book focuses on understanding the data through data interpretation and statistical interpretation.

Everyone may think that their way of DNA analysis is correct. However, misinterpretations of some fundamental principles have given rise to a variety of approaches being undertaken in labs today, some of which are not optimal, or even border on being incorrect for certain scenarios of use. Unfortunately, often times the approaches taken for interpretation are subjective, and therefore become the weakest part of the overall DNA typing process. I have written this book because I believe that a better understanding of fundamental principles will aid consistency and quality of work being performed in forensic DNA laboratories around the world.

In February 2009, the U.S. National Academy of Sciences released a report entitled “Strengthening Forensic Science in the United States” (NAS 2009). The report emphasized that good (forensic) science includes: (1) valid and reliable methodologies, and (2) practices that minimize the threat of bias in data interpretation. My Methodology volume demonstrates that valid and reliable methodologies can be achieved with forensic DNA typing. This Interpretation volume seeks to help minimize the threat of bias in data interpretation.

Good science takes time and effort to do well. It is worth noting that some measurements and interpretations are more reliable than others. Hence, uncertainty in measurements and interpretation should be reflected in the reports generated in a forensic case investigation. As will be described throughout this book, it is important that assumptions made during the interpretation process be documented and conveyed as clearly as possible. This documentation will aid those individuals reviewing the lab report to appropriately assess the results obtained and the conclusions drawn. It is important for analysts to offer what they know from the data obtained in a case in a fashion that is as clear and unbiased as possible.

That being said, I recognize that there are two areas of forensic DNA interpretations that are particularly challenging: (1) low-level DNA samples where sensitivity is an issue, and (2) complex mixtures where specificity is an issue. In other words, how much DNA is needed to obtain a reliable result and how well can the number of contributors to a sample be estimated to limit the uncertainty or ambiguity in the conclusions drawn. Chapters 7 and 13 will discuss some potential approaches to handling difficult interpretations. Unfortunately, in many situations involving complex results where uncertainty in the interpretation is large, the only scientifically responsible conclusion is “inconclusive” to avoid the chance of inappropriately including or excluding a potential contributor from an evidentiary result.

The Interpretation Process

An overview of the components and processes involved in data interpretation are illustrated in Figure 1.2. A sample data file contains time and fluorescence information for a PCR-amplified sample along with an internal size standard. The sample data file, which has a file extension of .fsa or .hid, is loaded into genotyping software along with an allelic ladder data file containing the same internal size standard to enable the sample and allelic ladder results to be correlated. Along with the allelic ladder sample, STR kit manufacturers provide a computer file specific for each STR kit containing bins (that define the allele repeat number for each STR locus) and panels (that define the STR loci present in the kit). When combined with the allelic ladder data file, bins and panels provide genotyping software with the capability to transform DNA size information into an STR allele repeat number for each observed peak.

Laboratory Protocols to Aid Interpretation

Laboratory protocols, which are often referred to as standard operating procedures (SOPs), are step-by-step instructions used to provide a consistent framework to gather and interpret information from analyzed samples. As part of a quality assurance system, accredited forensic laboratories will have written SOPs. Laboratory personnel are trained to understand and follow their laboratory-specific SOPs.

As will be described in more detail in Chapter 2, laboratories define parameters as part of their SOPs that act as thresholds within the genotyping software to filter information and aid analyst decisions that are made in determining the final sample DNA profile. These SOPs should be created based on validation data and then verified to work properly with control samples before being put into routine use.

A primary purpose of SOPs is to provide consistent results across DNA analysts within a laboratory as well as across cases analyzed by the same DNA analyst. The hope is that by following well-defined directions in a laboratory’s SOP, the same result can be obtained on a particular DNA sample by any qualified analyst or data reviewer.

FIGURE 1.2 Overview of DNA interpretation process illustrating that sample data files, at least one allelic ladder data file, and information from laboratory SOPs are entered into genotyping software. Analysts (or expert system software) review the information from the software to produce the final sample DNA profile.

Decisions during Data Interpretation

An analyst must make decisions about whether electrophoretic data...

Erscheint lt. Verlag	28.7.2014
Sprache	englisch
Themenwelt	Recht / Steuern ► Strafrecht ► Kriminologie
Themenwelt	Sozialwissenschaften
ISBN-10	0-12-405854-X / 012405854X
ISBN-13	978-0-12-405854-5 / 9780124058545

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