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’ (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?
One of the key elements in the logical approach to evidence evaluation are the propositions used for the evaluation. They are, in a certain sense, the most important part of the whole process. At the same time, they are also one of the least understood.
Today’s post explores the concept of propositions. I will attempt to describe what they are, how they are used, why we don’t change them once set and why they matter so much, among other things… all from the perspective of forensic document examination (and other forensic disciplines).
What is certification? In my opinion professional certification is a designation that indicates the holder of the certification has appropriate and adequate qualifications to do some particular, generally well-defined, job or task. As an example I am a forensic document examiner and I have received professional certification from the American Board of Forensic Document Examiners, Inc.
An internet search for ‘certification’ produces a huge list of possibilities, with more such programs being developed all the time as people become attuned to issues of quality and competency. Indeed, almost every profession has some type of certification and a few have several (consider all of the ‘certifications’ in the computing industry). Most, if not all, certification programs are aimed at improving the quality in a given profession by setting minimum standards for the job. The basic idea is that someone meeting or exceeding those standards will produce quality output on the job. Certification programs are generally created or are administered by a professional society, a college or university, or some private body set up expressly for that purpose.
Forensic Document Examination is no exception so it may be worthwhile discussing certification options as well as the pros and cons that I see for those options.
Some document examiners prefer to be called ‘forensic’ document examiners while other prefer the term ‘questioned’ document examiner. Is there any difference?
The simple answer is ‘no’; there is no real difference. Historically, the term used was “questioned document examiner” but in the last 15-20 years, “forensic” has become a much more common adjective applied to almost any (scientific) endeavour intended for court purposes. Just to add another variation to the discussion, when I began working in this field in the mid-1980’s my colleagues were called “examiners of questioned documents”.