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Archival data from a Chicago field study were examined to determine how lineup structure, presentation, and documentation may unfold in police cases and to consider the uses or pitfalls of field data for evaluating lineup procedure (system factor) effectiveness. A primary question concerned the anomalous low rate of filler selections by eyewitnesses for Chicago lineups. These data alert researchers to 2 primary pathways for distortion of descriptive archival data. Biased lineup procedures (lineup structure and presentation) can move witnesses toward the suspect. Biased documentation can enhance reports that maintain the police presumption of the suspect-as-perpetrator. Both processes will diminish the precision and validity of lineup evidence and undermine evaluation of system variable effectiveness in police practice.