This is Part II of a study estimating the random match frequency of acquired characteristics in footwear.
This study, which serves as Part II of an investigation into the random match frequency of randomly acquired characteristics (RAC-RMF) in footwear evidence, encountered non-zero RAC-RMFs for shoes exhibiting at least one forensically-reliable RAC at a more frequent rate than any estimates previously reported. 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). 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. (Published Abstract Provided)