. "PART III: Implications for a National Reference Ballistic Image Database, 8 Experimental Evidence on Sources of Variability and Imaging Standards." Ballistic Imaging. Washington, DC: The National Academies Press, 2008.
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We were interested in determining whether the system reliably found this exact same image—differing only by acquisition at different times, possibly by different operators—in databases of different effective sample sizes (between 5,312 and 15,082) after the standard filtering. The Glock entries (NYSP03 and NYSP04) provided the opportunity to see where casings in the same manufacturer-supplied envelope related strongly to each other. And, on a follow-up visit to the NYSP Forensic Investigation Center, the makes and models for the top 10 results by both breech face and firing pin were recorded to see whether like models dominated the top rankings.
As with the NAS-labeled exhibits, the current IBIS system had no problem detecting the “needle”—a new instance of the same exhibit image—in “haystacks” of varying sizes, with one prominent exception. That exception was NYSP01, a “new” Beretta 9mm; because it is a rimfire weapon, image entry is done by manually tracing the region of interest (see Section 4–B.2), and a single ejector mark/rimfire impression score is returned by the system. For this exhibit, the image on file in CoBIS—acquired that same morning—appeared as the 137th ranked possible match; its score was 328, compared to the top score (to another Beretta pistol, entered in 2004) of 571. The effective sample size was 8,106, so the link from NYSP01 to itself was not in great danger of being excluded by the coarse comparison and 20 percent threshold steps. Visual examination of the surface images suggest a curious ridge-like structure in the rimfire impression that apparently registers differently under slightly discrepant lighting and orientation. NYSP02, the “archive” Beretta casing, encountered no such difficulty; the original image from 2004 was found in the #1 position with score 2,631, with the score dropping to 444 for the #2 entry.
For each of the Glock exhibits, the second casing in the manufacturer-supplied envelope could be found as a match in the top 10 by one of the marks. When NYSP03-2 was used as the reference, NYSP03-1 was returned as the #4-ranked entry on breech face (raw score 61, relative to a maximum of 64) but was not within the top 10 on firing pin. When NYSP04-2 was the reference, NYSP04-1 was the top-ranked potential match by firing pin (score 168, with an unrelated Glock scoring 163 as the #2 possibility) but dropped out of the top 10 on breech face. In all these cases, the demographic filtering by Glock firing pin gave an effective sample size of 12,353 casings.
For the remaining NYSP-labeled exhibits, the same-gun image was very comfortably returned as the #1-ranked entry on both the breech face and firing pin scores, with wide separations between it and the remaining entries. The top 10 lists for each exhibit are most populated by guns by the same manufacturer and the same model, except for the Kimber exhibits (NYSP07 and NYSP08), for which none of the less-than-30 Kimbers in the CoBIS system were returned as top-10 candidates by either score.