指纹证据似然比评价数据库的构建与评估

Construction and Evaluation of the Database for Likelihood Ratio Evaluation of Fingerprint Evidence

建立科学的指纹证据量化评价体系,尤其是将似然比统计学方法引入指纹鉴定意见的数字化表达,是当前法庭科学理论和实践研究的热点问题。构建科学有效的指纹证据似然比评价模型,需要丰富的同源和异源指纹数据库来获得具有稳定分布规律的似然函数,因而,同源和异源数据库的质量高低直接影响了似然比模型的性能优劣。本研究通过活体采集仪和录屏软件获取每枚指纹在不同变形方式下产生的1000枚以上变形指纹图像,200枚模拟现场指纹共得到20万枚同源指纹,进而构成同源指纹数据库;异源指纹数据库则选用公安实战中千万人级的十指指纹数据库。在此基础上,运用指纹自动识别系统进行查询比对,并评估比对得分数据。实验结果表明,不同变形方式的指纹数据差异较大;压力大小和捺印时间对指纹的比对得分影响很小。由对样本总量不同程度缩减后的子样本数据的统计分析结果可知,每枚指纹的同源样本数量在少至155枚时仍然能形成稳定的分布规律。因此,所构建的数据库中同源和异源指纹样本数量丰富,结构合理,具备形成稳定分布规律的数据基础,有助于后续似然比评价模型的建立。

The establishment of a scientific quantitative evaluation system for fingerprint evidence,especially how to introduce the statistical method of likelihood ratio into the digital representation of fingerprint identification,is a hot issue in the current theoretical and practical research of forensic science.The construction of a scientific and effective likelihood ratio evaluation model for fingerprint evidence requires rich same-source and different-source fingerprint databases to obtain the likelihood function with a stable distribution law,thus,the quality of the same-source and different-source databases directly affects the performance of the likelihood ratio model.By using the live-scan fingerprint collector and screen recording software to obtain more than 1000 distorted fingerprint images for each fingerprint in different distortion modes,a total of 200000 same-source fingerprints are obtained from 200 simulated fingerprints,which constitutes the same-source fingerprint database;and the different-source fingerprint database consists of ten million people’s ten-fingerprint database in policing practice.On this basis,the automatic fingerprint identification system is utilized for query and comparison,and the comparison score data are evaluated.The experimental results show that the fingerprint data of different distortion modes have significant differences;the degree of pressure and the impressing time have little effect on the comparison scores of fingerprints.From the results of statistical analysis of the total number of samples and the subsample data after different degrees of reduction,it can be seen that the number of same-source samples of each fingerprint can still form a stable distribution law when the number of fingerprints is as few as 155.Therefore,the database we built is rich in the number of same-source and differentsource fingerprints,reasonable in structure,and has the data basis for forming a stable distribution law,which is helpful for the subsequent establishment of the likelihood ratio evaluation model.