This study examined rater effects on essay scoring in an operational monitoring system from England's 2008 national curriculum English writing test for 14-year-olds. We fitted two multilevel models and analyzed: (1) drift in rater severity effects over time; (2) rater central tendency effects; and (3) differences in rater severity and central tendency effects by raters' previous rating experience. We found no significant evidence of rater drift and, while raters with less experience appeared more severe than raters with more experience, this result also was not significant. However, we did find that there was a central tendency to raters' scoring. We also found that rater severity was significantly unstable over time. We discuss the theoretical and practical questions that our findings raise.