Measurement Uncertantity in Latents

[josher89]

Can anyone provide a reasonable and justifiable reason why I should be worried about measurement uncertainty in the field of latent prints? This might be a better question for the AFQAM board but I'm not a member. My reason for there not being a necessity for traceability and measurement uncertainty is this:

If I'm mixing up a reagent, and I'm off by a few 10th's of a gram for something, I've never noticed a significant over- or under-production of the intended result of the reagent (amino acid stains, lightning liquinox, etc.). Also, if I'm calibrating an image 1:1 for AFIS or NGI submission, I've been off by as many of 25 or 30 pixels per cm and have had no issues with getting hits (I know this because I varied my results slightly on purpose).

So, do I need a NIST-traceable ruler to calibrate my lab rulers when I'm doing 1:1's or do I need a NIST-traceable weight or volumetric cylinder to calibrate my balance or graduated cylinder when I am making ninhydrin? I can't find a justifiable reason why I would need that but I'm sorta asking people to change my opinion on it with justifiable responses. I understand that with accreditation, if you say you are doing something, then you should be doing it but if you don't need to, should you be doing it? I don't want to add extra layers if I don't need to.

Thanks, and I'll take answers off the air.

[John Vanderkolk]

While attending a seminar a few years ago, a panel was presenting measurement uncertainty as related to firearm and tool marks. When there was an opportunity for questions, I asked something like, 'when will measurement uncertainty within the human brain take place?'

Nobody on the panel answered my question. They deferred to a member of the audience, who is aware of measurement uncertainty, and he responded something like, 'John, we will have to get back to you on that.'

I know the responder and he has not yet gotten back to me with an answer. I am sufficiently certain, it will be a while. Somebody else might need to be around to receive the answer for me. After all, when we comparatively measure imperfect details of the features of the source that are recorded in finger prints, tool marks, shoe prints, etc., we are comparatively measuring imperfectly recorded details that vary in appearances, among each other and back to the features of the source. Since there are no such things as perfect matches, all of our comparative measurements are uncertain. Plus, the measurement uncertainities vary among all the comparative measurements we make. When do we get beyond many uncertain varying comparative measurements to reach an aggregate of sufficient;y certain measuerments?

That question I asked the panel is one of my favorites. Sounds like a research program: Calibrating the examiner's brain for determining comparative measurement uncertainty. Then, bringing all the uncertain measurements together to reach sufficient certainty.

So Josher89, I do worry about measurement uncertainty in latent prints, as every comparative measurement we make is uncertain. Is my reason reasonable and justified?

JohnV

[Peter Griffin]

I sent this post to my friend who is a Senior Criminalist in the Quality Assurance Unit for California DOJ. His main job is ensuring all laboratories within the system are in compliance with accreditation requirements; primarily focused on measurement traceability and measurement uncertainty. He is also an ASCLD/LAB Technical assessor for drugs, clan labs, fire debris, and measurement uncertainty, and teaches classes on measurement traceability and measurement uncertainty.
This was his opinion on the matter:
____________________________________________________________________________

Here’s my opinion…

Measurement Uncertainty in Latent Prints

I’m assuming that your laboratory is accredited to ISO/IEC 17025 standards either through ASCLD/LAB, A2LA, or ANSI/FQS. In my opinion, Measurement Uncertainty is not applicable to the discipline of latent prints as there are no real critical measurements being made. The closest critical measurement may occur when using a ruler and computer system to produce an accurate 1:1 photograph. However, knowing the uncertainty of measurement in this instance is not as important as simply ensuring the accuracy is adequate for your purposes. Most importantly estimating your measurement uncertainty for these things is not a requirement made by ASCLD/LAB (including their Measurement Uncertainty Policy), A2LA, or ANSI/FQS. The only way this could be required is if your Quality Management System made it a requirement.

Measurement Traceability in Latent Prints

Reagent formulas that involve weight and volume DO NOT need to have measurement traceability associated with them. There is no sense in having an ISO 17025 calibration that includes measurement traceability to the International System of Units by a vendor accredited by an internationally recognized accrediting body. As you stated, the accuracy required is not critical. However, the laboratory should still ensure that the balance is accurate to the level the laboratory requires by using a combination of regular performance checks and/or commercial (lower cost/quality) calibration services.

Similarly, latent print photography DOES NOT need to have measurement traceability either. The rulers you use in photographs do not need to be calibrated for the purposes of measurement traceability. The bigger issues with measurement accuracy in photography will most likely come from image plane not being parallel to the film/sensor plane in the camera AND/OR impression/latent print and ruler are not in the same plane. Inaccuracies in the ruler itself will most likely contribute very little to problems with 1:1 image accuracy – and therefore do not require a calibration/certification of accuracy. However, the laboratory should still ensure that the ruler and computer system used to make 1:1 images work adequately by checking them prior to casework in some manner. In my experience, you cannot assume that a measuring device is accurate because the manufacturer provides a “certificate.” I have seen instances of Class A glassware received from the manufacturer, which do not meet Class A or even Class B standards.

Regardless, it is most important to note that measurement traceability for the latent print discipline is not required by the accrediting body nor the international standard. Traceability is required essentially when “the measurement result has a significant impact on the final test result.” It’s primarily reserved for quantitative analysis since the final test result is typically an accurate measurement. Regardless, those with technical responsibility within your own laboratory (with knowledge of the common practices in the scientific community) get to decide whether or not they think Measurement Traceability should be required for the measurements that are being made in the laboratory. As you stated, if your laboratory requires it, then you must do it. But I think you are on solid ground to not require measurement traceability or measurement uncertainty.

Of course, the above information is all my opinion and interpretation. Your laboratory management, Quality Assurance Manager, or even the accrediting body assessors may have a completely different perspective! That I can guarantee. Ha ha… Feel free to contact me directly if you want to talk about any of this further.

Josh

Joshua S. Spatola, Senior Criminalist

California Department of Justice
Bureau of Forensic Services
Quality Assurance Unit
1300 I Street, Room 1150
Sacramento, CA 95814
916.322.7140 (desk)

[josher89]

@John,

I absolutely appreciate your response to my question and I think that you hit the nail on the head. I do think that while I would never want to take quantifiable measurements of the latent print itself (for the reasons you've stated), I do know that some would argue that because temperature and humidity affect the measuring device (as does the quality of the manufacturer of the ruler), I don't think those changes are significant enough to have a significant impact on the final test result. As far as calibrating my brain, I would not be a great candidate for that so count me out. I'm terrible at golf and I blame it on my inability to judge distances so I'm always grabbing the wrong club, no mater where the ball goes!

@Josh (via PG--could you pass this on to him please),

Thank you for your response. It was way more eloquently put than I could ever have done but does confirm what I believe. I fully realize that so many labs are doing things because they think accreditation is telling them to do so (or because the QM is telling them to do so because that's how they interpret the standard) when it may not be necessary at all. That's where I'm trying to separate the wheat from the chaff. If I can avoid doing all of these unnecessary things, I can take the burden off of the people I work with to make this an easier transition.

I will add your contact info to my list in the event that something else comes up so I appreciate your offer.