Preparing for Expert Witness Testimony on Probabilistic Genotyping

With approximately half the forensic laboratories in the United States either using a probabilistic genotyping system or in some stage of implementation, cases processed using this method are now making their way through the court systems. Additionally, probabilistic genotyping has been used at the international level for nearly a decade. As you (as a scientist) make the adjustment to reporting probabilistic genotyping analyses, you must also transition to explaining these same results to judges, attorneys and juries.


Earlier this month, Rachel Oefelein and Samantha Wandzek of DNA Labs International (DLI) addressed potential questions and response options typically covered in both direct examination and cross examination of expert witnesses testifying to probabilistic genotyping during a webinar.


Written by: Tara Luther, Promega



Samantha Wandzek, Validation Manager, starts the webinar with recommendations on how to prepare for direct examination. She prefaces the conversation with, “There is no one right answer. It’s about finding your comfort level.”


She stresses that pre-trial preparation is one of the most important aspects for those who haven’t testified on probabilistic genotyping. Samantha advises meeting with your attorney to discuss how comfortable they are with probabilistic genotyping. She notes that some attorneys like to be well-versed in the subject matter and ask that Samantha provide additional resources, such as links to websites, questions to ask, study material, etc., while other attorneys prefer that she be the expert on the software used and be prepared to explain to a jury. Samantha also recommends asking the attorney how in-depth they’d like her to go with her testimony, and notes that this could depend upon the defense attorney on the case.


Samantha also recommends that you do your homework and know the following: how many times has software been presented in court before in that state and have there been any Daubert or Frye hearings in the past? She also advises re-familiarizing yourself with the case. Were there multiple statistics in the case? What contributor was the comparison sample assigned to for STRmixTM? Samantha notes that this second question can become very important in gun cases where a defense attorney may argue secondary gun transfer. She stresses that it’s important to bring all this up during pre-trial so that your attorney knows any limitations of the sample that you had to work with.


One question that is almost always asked is “What is probabilistic genotyping?” Samantha says that she doesn’t even use the words “probabilistic genotyping” a lot of the time. She continues, “When I’m talking about STRmixTM, I usually say ‘STRmixTM, and then define it as a DNA profile evaluator and statistical tool.” She has some questions you should consider: How experienced are you, and how comfortable are you in answering additional questions from the defense attorneys? She advises, “If you’re weary about answering more in-depth questions from the defense, you might want to have additional prepared questions on your direct examination.”


Rachel Oefelein, Quality Assurance Manager, says she prefers to use an analogy when explaining probabilistic genotyping, such as the “hot and cold game”. An example she gives is, “Look at this courthouse we’re in. It’s a very large building. The true answer that explains the evidence best is somewhere in this building. Your first guess may not be that good, but we’re going to guess millions and millions of times over until we get closer to that true answer.” She notes that it’s important to make eye contact with the jury as you’re using an analogy to ensure that they are following along.


Next, Samantha and Rachel discuss how to answer the question “What is a likelihood ratio?” Samantha says she’s straight-forward with her answer and says, “A likelihood ratio is a ratio with two probabilities giving a numerical value that shows strength of support for one scenario over another. In the case of DNA, in the simplest of terms, it’s saying, given the evidence, whether it is more likely that a DNA profile originated from Jane Doe rather than an unknown individual.” She clarifies that this answer would explain a single-source sample and could be altered as needed.


Rachel then describes how she’d use a breakfast analogy, saying, “Breakfast is something that a lot of people can relate to. You have three components to your breakfast, and I know that, but if I say that it’s 30 trillion times more likely, given the evidence, that a poached egg was a component of your breakfast, as opposed to three random breakfast items, that would hold a lot of weight.” She relates that to a three-person mixture by stating, “If I’m telling you it’s x times more probable that the evidence is explained with this individual as opposed to three random individuals, there’s a lot of weight to that evidence.” Both Samantha and Rachel note that it’s important for you, as the one testifying, to know the verbal scale used in your lab and how to explain this to a jury.


Another question that is frequently asked in court is “How do you know the results are accurate?” Samantha recommends answering this way, “The software is just a tool used by an analyst. There are many different diagnostics that can be evaluated to ensure the software is accurate. In addition, the results should be intuitively correct. As an analyst, I can check to see if the profiles being determined and the statistic generated makes sense with the observed data.” Rachel adds that she often tells the jury that, “Diagnostics are like taking your temperature. It’s how well the computer software program is able to explain the evidence.”


Rachel then discussed what you should do if there is an admissibility hearing. She advises analysts are prepared. What does that look like? She recommends doing the following:

  • Know what is being argued.
  • Know what standards need to be satisfied for the court. Daubert? Frye? Something else?
  • Mocks are almost always harder than the real thing, so practice!
  • Read transcripts from past hearings that have been held. You may like how someone else has answered questions better.
  • Set up a pre-hearing meeting with the attorney
  • Review any previous deposition transcripts on the case.
  • Know real facts such as: internal validation studies, number of publications, actual publications to support evidence, SOP, what other laboratories use the same technology, etc. You don’t need to memorize this information as it can be provided as evidence submitted before the hearing so that you can refer back to it.


Rachel closed with some advice on cross-examination. She lists some important things to remember, such as don’t take any questions asked personally – it’s the attorney’s job to ask them, and to stay calm and take moments to collect yourself and breathe. She also emphasizes that you shouldn’t allow yourself to be bullied. If a defense attorney fires off questions rapidly, she recommends saying, “May I answer your first question?” and then, “What was the second question?” She also notes that it’s important to correct yourself as quickly as possible in the event you misspeak, as it’s your job to represent the evidence.


She also notes that some defense attorneys may not be as familiar with the software or may try and trip you up with their questions. In either case, she recommends clarifying the question before answering. Rachel also advises talking with your attorney before the trial to outline any important details you’d like to cover during testimony. This way, your attorney may be able to bring them up during a re-direct.


In the recording of the webinar, Rachel details real-world questions that were asked of her during cross examination and describes how she would answer these questions. She and Samantha also elaborate on how to explain diagnostics to a jury. If you find that you could use some additional preparation for testifying on probabilistic genotyping, I’d highly recommend you watch the full hour, including questions from the live audience. Rachel will also be participating in the Probabilistic Genotyping Expert Testimony: Communication, Challenges, and Examples workshop held at ISHI this year.