Every ASQDE meeting is worth attending. They are great fun with lots of useful and interesting content. Unfortunately, I could not make it to the 2016 ASQDE conference held in Pensacola, Florida. Nonetheless I managed to participate, albeit via Skype.
One of the activities at the conference was a panel discussion discussing “Approaches to Evaluation and Reporting of Expert Evidence” and I was invited to participant with three other people. It was a very interesting session…
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The concepts of ‘prior odds’, a.k.a., prior probabilities or simply priors, and ‘posterior odds’ come up in most discussions about the evaluation of evidence. The significance and meaning of both terms becomes clear when viewed in the context of a “Bayesian approach”, or the logical approach, to evidence evaluation. That approach has been discussed at length elsewhere and relates to the updating of one’s belief about events based upon new information. A key aspect is that some existing belief, encapsulated as the ‘prior odds’ of two competing possibilities or events, will be updated on the basis of new information, encapsulated in the ‘likelihood-ratio’1 (another term you will undoubtedly have seen), to produce some new belief, encapsulated as ‘posterior odds’ about those same competing possibilities.
But what precisely do these terms, ‘prior odds’ and ‘posterior odds’, mean and how do they relate to the work of a forensic examiner?
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Several of the posts on this blog relate to the logical approach to evidence evaluation; aka, the coherent logical approach, or the likelihood-ratio (LR) approach. In my opinion, it is the best way to evaluate evidence for forensic purposes no matter what type of evidence is being discussed. I say “best” because it is simple, logically sound, and relatively straight-forward to apply in forensic work. It helps to promote transparency through the application of a thorough and complete evaluation process (all points I have explained in other posts).
The reality is, however, that this approach is still not well understood by forensic practitioners, nor by members of the legal profession.
I hope that in time, and with education, that will change. Several workshops I have presented have been aimed at helping examiners understand what it really means, how it works, the philosophical basis behind the approach as well as the need for and benefit of doing things that particular way. It really does work to the benefit of both the examiner and their ultimate client, the court.
One recurring issue at these workshops relates to the very basic and fundamental concept of what the term “Bayesian” means. For various reasons, but mainly just misunderstanding, many people in the forensic document examination community hold the term “Bayesian” in negative regard. When the word ‘Bayes’, or any of its many derivations, come up in the conversation eyes glaze over while heads sag ever so slightly. And those are the positive people in the crowd.
I find such reactions understandable, but unfortunate. The fact is that an understanding of the term is beneficial for anyone interested in how it might be applied in a forensic evidence context, whether or not one chooses to do so. Indeed, for myself the answer to the question posed above — when is a Bayesian not a Bayesian? — lies in knowing how the overall Bayesian philosophy and theorem (or rule) differs from the more constrained and limited logical approach to evidence evaluation. These two are not the same or even close to equivalent.
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Okay, determining the ‘best’ of anything is always a challenge. It is, in almost every instance, a highly subjective decision based on some set of appealing features or characteristics… appealing to the person making the determination, of course. And, because this is my blog, that person happens to be me. In fairness, there are a number of authors who have written extensively on the topic: Osborn, Ellen, Hilton, Harrison, Hilton, among others (and I apologize to those I have left off this list). I have read all of those textbooks (including most editions) and each has its strengths and weaknesses. Nonetheless, in my opinion the best general textbook written to date on the topic of handwriting identification was done by co-authors Roy A. Huber and A.M. (Tom) Headrick, both long-time document examiners in the R.C.M. Police laboratory system.
That textbook is Handwriting Identification: Facts and Fundamentals.1Read more
When someone “opens a can of worms” it usually spells trouble. For many people, that phrase evokes a powerful image of a writhing mess of worms escaping from a previously-sealed, but now opened, can or container. With the result of such action being serious problems for the owner of said can, often problems of an unanticipated or uncertain nature. In the context of our work as Forensic Document Examiners, I sometimes hear this coming up in discussions of how to handle questions on the stand. The advice goes along the lines of ‘keep your answers simple and say as little as possible in order to limit any opportunity for questions from the other side.’
It is suggested that lengthy or complex answers will only lead to more questions and more discussion. The latter are the proverbial “can of worms” that one must strive to avoid opening.
That makes little sense to me.
The 2014 ASQDE–ASFDE conference included an interesting panel discussion with the title “Conclusions… Signature and Handwriting Conclusion Terminology and Scales”. I was fortunate to be able to take part, albeit only remotely via Skype.
The abstract for the session was as follows:
A current and global issue in our field is the topic of conclusion terminology and conclusion scales, particularly in respect of signature and handwriting conclusions. It is an important yet difficult topic to address because, while there is some commonality in the conclusion scales used in different geographical regions around the world, within a number of geographical regions there are multiple scales in use. It is for this very reason that it is also a topic in great need of discussion and there is a strong argument that we should attempt to reach a consensus (even if the result is that we agree to disagree).
This panel discussion is a collaboration of insights from numerous colleagues in our field in person, via Skype and in writing from private and government laboratories in geographical regions across the Americas, Australia, Asia, Africa, the Middle East and Europe.
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This year’s International Conference on Forensic Inference and Statistics (ICFIS) is being held at Leiden University in the Netherlands. 
ICFIS conferences are always very good and this is the 9th such event. I am hoping to attend to present my thoughts on the topic of education relating to the logical (a.k.a. likelihood-ratio or LR) approach to evidence evaluation. Over the last few years I have given several one and two-day seminars and workshops on this topic, mainly for Forensic Document Examiners (FDEs) though the subject matter relates to all disciplines equally. Those workshops have been great and provided a relatively unusual opportunity to learn about how fully trained examiners come to grips with a complicated and difficult topic. One that is fundamental to FDE work.
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This year the Annual General Meeting of the American Society of Questioned Document Examiners (ASQDE) 
is being held in Indianapolis, Indiana on August 24 through 29, 2013. In keeping with the theme, “Demonstrative Science: Illustrating Findings in Reports and Court Testimony”, I will be presenting a one-day workshop entitled “Conclusion Scales and Logical Inference” on Sunday, August 25.
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Forensic scientists, individually and as a group, want to be completely logical, open and transparent in their approach to the evaluation of evidence. Such an assertion is unquestionable. Further, I am sure that most document examiners believe this is exactly what they are achieving when they apply the procedures outlined in various traditional textbooks or the SWGDOC/ ASTM standards; for example, the SWGDOC Standard for Examination of Handwritten Items. Given the very understandable desire to be logical, I find it strange that so many people have a negative attitude towards anything and everything “Bayesian” in nature. After all, a logical approach to evidence evaluation that conforms to the overall Bayesian philosophy or approach is, quite literally, the embodiment of logic (more specifically, probabilistic logic).
Read moreThe terms accuracy and precision are often confused or misunderstood. But every scientist, forensic or otherwise, should understand what they mean. In simple terms, ‘accuracy’ relates to how closely the value comes to the real score or true value (being ‘on target’). ‘Precision’, on the other hand, relates to the consistency of the value in repeated testing. Any given test, statistic or process may produce results that are one or the other, both or neither.
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