[Servant of the 9]

This is a matter of tolerance stacking, something that always must be avoided in science and engineering.

Say you are matching to an accuracy of 0.0000001

Sample 'A' is 0.1234567
Sample 'B' is 0.1234568

They match within the margin of error.

Sample 'B' is 0.1234568
Sample 'C' is 0.1234569

They match within the margin of error

By the FBI standards

Sample 'A' = 0.1234567
and
Sample 'C' = 0.1234569
Difference = 0.0000002
are a 'match' by data chaining even though they differ by twice the margin of error.

With a longer chain, you can make any two samples match.
In other words, it is pure pseudoscience.
They are making it up as they go along.

So9

[You Gotta Be Kidding Me]

"This is a matter of tolerance stacking, something that always must be avoided in science and engineering. Say you are matching to an accuracy of 0.0000001 Sample 'A' is 0.1234567 Sample 'B' is 0.1234568 They match within the margin of error. Sample 'B' is 0.1234568 Sample 'C' is 0.1234569 They match within the margin of error By the FBI standards Sample 'A' = 0.1234567 and Sample 'C' = 0.1234569 Difference = 0.0000002 are a 'match' by data chaining even though they differ by twice the margin of error. With a longer chain, you can make any two samples match. In other words, it is pure pseudoscience. They are making it up as they go along."

I once asked a coworker why the International Olympic Committee kept changing the rules on Olympic basketball competition (no "allyoops" allowed, no vertical jumps over 30", no ...black guys allowed would be next, etc.). My coworker replied: "They'll keep changing the rules until the US teams lose. Europeans are tired of having their teams lose."

You're right. I think that happened at the FBI forensic labs, as well. "Close enough" ain't good enough, when it comes to evidence.

[FreePaul]

I've often heard the term "Close enough for government work." I guess that applies here. Crime labs, that of the FBI and local authorities, too often are charged with producing evidence to fit conclusions of police and prosecutors. Seems they do this all too well.