When it comes to the level of certainty, there is no one that is completely right or wrong that has posted here on this subject. One of the best articles, that include some research and testing, can be found at:
[www.researchgate.net]

This article gives information on the subject that is important to the individual that believes or does not believe in using the term with a degree of certainty.

I was talking with a person last night about this subject. What was related to me I thought was interesting. It showed how an investigator can get caught up it the term to where in trying to defend it he ended up not being able to testify. He went along the lines of the discussion that has taken place here on the site. He said he used the scientific method found in 921. He was then asked if that methodology produced a level of certainty as to the correctness of his opinion. He then when into the falsification discussion of the scientific method. He was asked if because that methodology could not say his opinion was correct. He was then asked if he had a level of certainty that his opinion was correct. He answered yes. Of course, the next question was what the level was. The investigator reverted to 921 and said it was probable. He was then asked what percentage does probable mean. He said more than 50 precent. He was then asked does that mean you have almost a 50 precent chance of your answer being wrong. In trying to get out of this he said his opinion was more than the 50 precent. Next question was what percentage it is. This questioning went on for a while to the point the judge cut off the questioning and ruled the investigator was not qualified to give the opinion. The judge’s reasoning was that if he could not explain how his methodology would result in a correct answer as to how the fire started and he seem not wanting to give it a high degree of certainty of being correct that his testimony is nothing more than speculation.

What would be best is to remove the line of questioning. As to the level of certainty this was done by the Justice Department when they instructed that the term level of certainty was to no longer appear in reports. Unfortunately, this did not address the falsification issue in the scientific method. I’m sure we will see that challenged somewhere soon. Attorneys are getting smarter by the day and I’m listening to experts as to how to challenge other experts.

Here is something you may want to consider and be able to explain before attempting to give testimony. Ask yourself these questions. You may even want to practice the answers. In formulating my opinions, did I consider all relevant facts? Do I have adequate understanding of pertinent scientific principles used in the formulation of the opinion? Did I use methods of assessment that are appropriate, reliable, and valid? Can I explain the methodology used to conduct the investigation and reach a conclusion? Are my inferences, assumptions, and conclusions reasonable and defensible?

Jim Mazerat
Forensic Investigations Group