This study compares the effects of data-driven assessor training with schema-driven assessor training and control training. The sample consisted of 229 industrial and organizational psychology students and 161 managers who were randomly assigned to 1 of these training strategies. Participants observed and rated candidates in an assessment center exercise. The data-driven and schema-driven assessor training approaches outperformed the control training on all 3 dependent variables. The schema-driven assessor training resulted in the largest values of interrater reliability, dimension differentiation, and accuracy. Managers provided significantly more accurate ratings than students but distinguished less between the dimensions. Practical implications regarding the design of assessor trainings and the composition of assessor teams are proposed.