The aim of this study was to estimate random match frequency of randomly acquired characteristics (RAC-RMF) for laboratory-simulated crime scene impressions. Part I of this investigation reports this ...metric using a dataset of more than 160 questioned impressions created in blood and deposited on tile. A total of 759 RACs were identified in the blood impressions and compared to RACs with positional similarity in test impressions from 1,299 unrelated outsoles. Geometric similarity was quantified using a combination of visual comparisons and mathematical modeling based on percent area overlap. Results indicated that RACs in blood impressions were typically smaller, and therefore exhibited a two-thirds increase in the number of indistinguishable pairs compared to their mated test impressions. For shoes contributing at least one RAC, relative RAC-RMF values ⩾ 0.0008 were encountered at a rate between 3.4% and 34% for the blood impressions examined in this study. Part II of this investigation provides analogous results based on dust impressions deposited on paper and tile. Although the results in Part I and Part II are specific to randomly acquired characteristics and do not translate into an impression-wide RMF estimate, this research shows that RACs in questioned impressions of the type expected in casework co-occur in position and geometry with RACs in non-mated test impressions. Since theoretical models have traditionally been the basis for estimating RAC-RMF in footwear, the overall contribution of this research to the forensic footwear community is a calibration of this estimate based on empirical data.
This study serves as Part II of an investigation into the random match frequency of randomly acquired characteristics (RAC-RMF) in footwear evidence. In Part I, RAC-RMF was estimated in a dataset of ...laboratory-simulated crime scene impressions deposited in blood. For Part II, a second dataset was created composed of impressions deposited in dust on paper or tile, with the latter lifted using gelatin or Mylar film. A total of 1,513 RACs were identified from more than 160 dust impressions and compared to RACs with positional similarity in test impressions from 1,299 non-mated outsoles. RACs of any size deposited in dust exhibited a 31% decrease in shoes with non-zero RAC-RMFs as compared to their mated test impressions, while those deposited in blood exhibited a 45% increase. When only considering shoes with at least one RAC deemed forensically-reliable (length ⩾ 2.8 mm), 3.1% of shoes contributing dust impressions and 3.4% of shoes contributing blood impressions exhibited relative RAC-RMFs at a value ⩾ 0.0008. Although each dataset resulted in a comparable rate for encountering non-zero RAC-RMFs, the estimate for dust was based on twice the number of RAC comparisons (154,477) than those performed when assessing blood (77,566). While these results are considered specific to the non-mated impressions and methods of analysis described herein, and continued work is required before rates can be fully understood and reported in forensic casework, this study encountered non-zero RAC-RMFs for shoes exhibiting at least one forensically-reliable RAC at a more frequent rate than any estimates previously reported.
•Randomly acquired characteristics (RACs) were compared on unrelated outsoles.•2,181 RAC pairs with positional and geometric similarity exist on 1,300 outsoles.•Random match frequency values ranged ...from 0 out of 1,299 to 49 out of 1,299.•32% of outsoles did not share any indistinguishable RACs with another outsole.•Up to five indistinguishable pairs were shared between two unrelated outsoles.
When analyzing footwear impression evidence, a significant task of the forensic examiner is to determine if a questioned impression could have originated from a known shoe. To form this opinion, examiners typically evaluate the similarity, quantity, and quality of shared class characteristics and characteristics of use. Since these criteria are developed through training and experience, and therefore purported to be subjective in nature, the opinions formed regarding footwear evidence can be misunderstood. One way to mitigate this criticism is to complement casework with research that includes quantitative analyses. The aim of this study was to estimate random match frequency of randomly acquired characteristics (RAC-RMF) in a research database comprised of 1,300 outsoles with more than 80,000 RACs. Based on a combination of visual comparisons (>91,000) and mathematical predictions (>3.8 million), results indicate that 32% of the outsoles in this dataset do not share any indistinguishable RAC pairs with each other, while 19% possess RAC-RMFs of 1 out of 1,299. At the other extreme, the maximum RAC-RMF observed was 49 out of 1,299. These results are based on high-quality test impressions, human assessments, and a single quantitative similarity metric, so they are considered specific to this dataset and method of analysis. Results could differ in other databases and with impressions of lower quality, and therefore should not be extrapolated to casework. Despite this limitation, the results provide a point of reference for how often RACs may repeat in position and geometry on non-mated outsoles, therefore forming the basis for future research.
When analyzing footwear impression evidence, one of the goals of an examiner is to determine if an exemplar shoe could be the source of an impression found at a crime scene. This opinion is based on ...an assessment of the similarity of class characteristics and randomly acquired characteristics (RACs) between the known and questioned impressions, as well as the rarity of the observed characteristics. The primary aim of this research was to estimate the random match frequency of randomly acquired characteristics (RAC-RMF) within a forensic footwear database to determine the frequency of RACs with geometric similarity occurring in the same relative position on unrelated outsoles, which could potentially increase the chance of an erroneous source association.RAC-RMF was estimated using high-quality test impressions of the 1,300 shoes in the West Virginia University (WVU) footwear database. Each impression in the database was sequentially held out and compared to the remaining 1,299 impressions to determine if unrelated shoes possessed similar RACs in the same relative locations. With over 80,000 RACs available for analysis, this resulted in nearly four million comparisons, which were performed using a combination of visual comparisons and predictions from a mathematical model based on a percent area overlap similarity score. Nearly 70% of the shoes in the database shared an indistinguishable pair with at least 1 out of 1,299 unrelated shoes, with a maximum RAC-RMF value of 49 out of 1,299 observed, and up to 5 indistinguishable RAC pairs shared between unrelated outsoles.A similar evaluation was performed on two simulated crime scene impression datasets each containing more than 160 impressions deposited in blood or dust, respectively. A total of 759 RACs were identified in blood impressions created on tile, leading to over 77,000 non-mated RAC comparisons between blood impressions and test impressions from 1,299 unrelated outsoles. RACs in blood impressions were smaller on average than their test impression mates, and therefore exhibited a 66% increase in the number of indistinguishable RAC pairs. Depending on RAC length, relative RAC-RMFs of at least 0.0008 were encountered at a rate between 3.4% and 34%. The dust impression dataset included impressions deposited on paper and tile, with the latter lifted using either gelatin or Mylar film and an electrostatic lifter. A total of 1,513 RACs were identified from all impressions, generating over 154,000 non-mated RAC comparisons. The RACs in dust impressions were often similar in size or larger than their known mates, leading to a 42% decrease in indistinguishable RAC pairs relative to mated test impressions. As a result, relative RAC-RMFs of at least 0.0008 were observed at a rate between 3.1% and 32%, despite twice the number of RACs available compared to the blood impressions. This contrast suggested that a liquid medium may erode RAC size, while a particulate medium maintains or possibly increases RAC size, thus influencing non-mated RAC similarity. However, no more than one shared indistinguishable RAC pair was observed between unrelated outsoles for either dataset, meaning that an average of four and eight distinguishable RACs were present for blood and dust impressions, respectively.This research provided estimates of RAC-RMF for a large database of high-quality test impressions as well as two datasets of simulated crime scene impressions. Analysis of these datasets demonstrated that RAC geometries do repeat on non-mated outsoles, and the rate at which this occurs within each dataset was quantified. Since theoretical models have traditionally been the basis for estimating RAC-RMF in footwear and the majority of empirical studies have reported RAC-RMFs at or near zero, the contribution of this research to the forensic footwear community is a calibration of this estimate based on empirical data from a larger sample of outsoles.
