Objective evaluation of similarity scores derived by Evofinder^® system for marks on bullets fired from Chinese Norinco QSZ-92 pistols

Figures & data

Table 1. Descriptive statistics for KM and KNM similarity scores.

Descriptive statistics	Slippage mark		LEA		GEA
	KM	KNM	KM	KNM	KM	KNM
Minimum	0.042	0.003	0.183	0.092	0.208	0.124
Average	0.595	0.352	0.589	0.181	0.600	0.252
Maximum	0.942	0.899	0.925	0.945	0.949	0.886
Standard deviation	0.138	0.049	0.089	0.019	0.116	0.034
Kurtosis	2.848	5.259	3.168	6.531	3.072	4.137
Skewness	−0.586	−0.236	−0.063	0.815	−0.318	0.671

LEA: land engraved area; GEA: groove engraved area; KM: known match; KNM: known non-match.

Figure 1. Boxplots of known match (KM) and known non-match (KNM) scores. LEA: land engraved area; GEA: groove engraved area.

Figure 2. Scatter plots of known match (KM) scores (indicated by red circles) and known non-match (KNM) scores (indicated by green circles) for slippage mark, land engraved area (LEA) and groove engraved area (GEA).

Table 2. Classification results for combined scores based on supporting vector machine (SVM).

	Ground-truth label
	Known match specimens		Known non-match specimens
Judged as matches	1 983		6
Judged as non-matches	2		2 982 086

Figure 3. Receiver operating characteristic (ROC) plots of similarity scores for slippage mark, land engraved area (LEA) and groove engraved area (GEA).

Figure 4. Tippett plot of score-based likelihood ratio (SLR) performance for slippage mark, land engraved area (LEA) and groove engraved area (GEA). The solid and dashed lines indicate true-H_p and true-H_d curves, respectively.

Table 3. Misleading evidence ratio for H_p (RMEP) and misleading evidence ratio for H_d (RMED) value of slippage mark, land engraved area (LEA) and groove engraved area (GEA).

	Slippage mark (%)	LEA (%)	GEA (%)
RMEP	3.5409	0.0074	0.8740
RMED	15.6171	0.1002	2.7068

Table 4. Ranges of scores and correlations of known match (KM) scores given the range of scores for various score-based likelihood ratio (SLR) ranges.

SLR	Slippage mark		LEA		GEA
	Number of correlations	Scores	Number of correlations	Scores	Number of correlations	Scores
(−∞, 0.1)	138	[0, 0.363)	2	[0, 0.281)	27	[0, 0.309)
[0.1, 1)	175	[0.363, 0.441)	0	[0.281, 0.293)	27	[0.309, 0.353)
[1, 10)	158	[0.441, 0.496)	0	[0.293, 0.305)	40	[0.353, 0.393)
[10, 100)	141	[0.496, 0.541)	1	[0.305, 0.323)	55	[0.393, 0.43)
[100, 1 000)	182	[0.541, 0.583)	3	[0.323, 0.34)	90	[0.43, 0.464)
[1 000, +∞)	1 191	[0.583, 1)	1 990	[0.34, 1)	1 756	[0.464, 1)

LEA: land engraved area; GEA: groove engraved area.

Table 5. Ranges of scores and correlations of known non-match (KNM) scores given the range of scores for various score-based likelihood ratio (SLR) ranges.

SLR	Slippage mark		LEA		GEA
	Number of correlations	Scores	Number of correlations	Scores	Number of correlations	Scores
[10, +∞)	9 870	[0.496, 1)	67	[0.305, 1)	3 908	[0.393, 1)
[1, 10)	95 722	[0.441, 0.496)	154	[0.293, 0.305)	22 240	[0.353, 0.393)
[0.1, 1)	1 077 936	[0.363, 0.441)	430	[0.281, 0.293)	161 275	[0.309, 0.353)
[0.01, 0.1)	1 406 066	[0.302, 0.363)	813	[0.272, 0.281)	1 032 239	[0.257, 0.309)
[0.001, 0.01)	386 845	[0.156, 0.302)	1 457 751	[0.180, 0.272)	1 716 220	[0.191, 0.257)
(-∞, 0.001)	5 653	[0, 0.156)	1 533 505	[0, 0.180)	55 838	[0, 0.140)

LEA: land engraved area; GEA: groove engraved area.