Putting your finger on the distribution of latent prints (the 30% palm estimate)

Back when automated fingerprint identification systems (AFIS) were originally expanded to become automated fingerprint/palmprint identification systems (AFPIS), a common rationale for the expansion was the large number of unsolved latent palmprints at crime scenes.

By Etan J. Tal – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=41152228

The statistic that everyone cited was a statistic that 30% of all latent friction ridge prints at crime scenes were from palmprints. Here’s a citation from the National Institute of Justice.

Anecdotally, it is estimated that approximately 30% of comparison cases involve palm impressions.

Note that the NIJ took care to include the word “anecdotally.” Others don’t.

It is estimated that 30 percent of latent prints found at crime scenes come from palms.

But who provided the initial “30% of latents are palms” estimate long ago? And what was the basis for this estimate? This critical information seems to have been lost.

By Apneet Jolly – originally posted to Flickr as Candy corn contest jar, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=10317287

Now I don’t have a problem with imprecise estimates, provided that the assumptions that go behind the estimate are well-documented. I’ve done this many times myself.

But sadly, any assumptions for the “30% of latents are palms” figure have disappeared over the years, and only the percentage remains.

Is there any contemporary evidence that can be used to check the 30% estimate?

Yes.

The blind proficiency study wasn’t blind regarding the test data

Latent print quality in blind proficiency testing: Using quality metrics to examine laboratory performance. https://lib.dr.iastate.edu/csafe_pubs/84/

A Center for Statistics and Applications in Forensic Science study (downloadable here) was published earlier this year. Although the study was devoted to another purpose, it touched upon this particular issue.

The “Latent print quality in blind proficiency testing: Using quality metrics to examine laboratory performance” study obviously needed some data, so it analyzed a set of latent prints examined by the Houston Forensic Science Center (HFSC) over a multi-year period.

In the winter of 2017, HFSC implemented a blind quality control
program in latent print comparison. Since its implementation, the
Quality Division within the laboratory has developed and inserted
290 blind cases/requests for analysis into the latent print comparison unit as of August 4, 2020….

Of the 290 blind cases inserted into casework, we were able to
obtain print images for 144 cases, with report dates spanning approximately two years (i.e., January 9, 2018 to January 8, 2020)….

In total, examiners reviewed 376 latent prints submitted as part
of the 144 blind cases/requests for analysis.

So, out of those 376 latent prints, how many were from palms?

The majority of latent prints were fingerprints (94.3%;
n = 350) or palm prints (4.9%; n = 18). Very few were joint impressions or unspecified impressions (0.8%; n = 3)….

The remaining 5 of 376 prints were not attributed to an anatomical source because examiners determined them to be of no comparative value and did not consider them to be latent prints.

For those who are math-challenged, 5 percent is not equal to 30 percent. In fact, 5 percent is much less than 30 percent. (And 4.9% is even less, if you want to get precise about it.)

Now I’ll grant that this is just one study, and other latent examinations may have wildly different percentages. At a minimum, though, this data should cause us to question the universally-accepted “30%” figure.

As any scientific institute that desires funding would proclaim, further research is needed.

And I’ll grant that. Well, I won’t grant it, but some government or private funding entity might.

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