Contrary to previous research on sequential ratings of student performance, this study found that professional essay raters of a large-scale standardized testing program produced ratings that were drawn toward previous ratings, creating an assimilation effect. Longer intervals between the two adjacent ratings and higher degree of agreement with other raters were associated with smaller assimilation effects. The effect was also found to create a small bias in favor of low-achieving groups of examinees. (Contains 3 figures and 2 tables.)