ISO 17025 requires laboratories to document how uncertainty was calculated. Find out ways to become more comfortable with your uncertainty calculations.

In this article, we will address some questions about calculating uncertainty by using trigger pull measurements as an example. You don’t have to be a firearms examiner to understand the article. In fact, we chose this example over other measurements common to forensic labs, like quantitative drug analysis, because trigger pull is a fairly straight forward measurement. For our experiment, we used the setup displayed in Figure 1.

The readout from the measuring device, a Lyman Electronic Trigger Pull, is depicted in the drawing in Figure 2. A careful reading of the manual tells us that the digital display reports pounds and ounces. So, the display is showing 5 lbs 11.5 oz. This measurement raises several questions. For example:

Question 1: If you took just one measurement and got the result in Figure 2, is it okay to report the trigger pull as 5 lb 11.5000 oz?

Most of you probably said “no” because you recognized that the trigger pull device does not give us any information about those trailing zeros after the 11.5.We don’t know what those numbers actually are.

Question 2: Is the right answer more like: Trigger Pull = 5 lbs 11.5 oz ± 0.05 oz?

If you answered yes, you are getting warmer, and this topic was covered in a previous Forensic Magazine article which can be found at www.forensicmag.com/articles.asp?pid=193 (Please note the hard copy of this article has a math error that has been corrected in the online version). However, as we will see in a moment, the real calculation of uncertainty is a little more complicated and interesting. Finally, let’s pose one last question.