Tested 39 baseball umpires and 39 undergraduates, using a computer simulation of 1st-base calls to determine the role of the prior entry phenomenon in close plays. Because umpires are directed to focus their attention on an auditory stimulus (ball hitting the glove), they may experience prior entry of the auditory stimulus, resulting in safe runners being called out. There were more errors when runners were just safe and safe because of a tie than when runners were just out, indicating that prior entry occurred in the simulations. This was more pronounced among umpires than controls.
Larsen, Janet D. and Rainey, David W., "Judgment Bias in Baseball Umpires First Base Calls: A Computer Simulation" (1991). Psychology. 4.