This research investigated cognitive processes affecting the accuracy of performance evaluation. One hundred and twenty-five nurses from three hospitals completed questionnaires measuring their cognitive categorization processes and then rated a videotaped nurse's performance. Results focusing on the match between raters' cognitive categories and rating scales indicated that: a) rating accuracy was related to the match between raters' cognitive category dimensions and rating scale dimensions, but b) not related to the match between rating scales and raters' general tendency to describe ratees in terms of behavioral or trait characteristics. Raters' ability to differentiate clearly between rating dimensions was related to accuracy and to halo in ratings. Finally, work experience was related to raters' cognitive systems while prior rating experience was important for accuracy. Limitations of the study and implications for future research are discussed.