“…The decision to categorize sets of observations such as item-score vectors as either fitting or misfitting is known as outlier identification (cf., Barnett & Lewis, 1994, p. 7). Recent examples can be found in certification testing (Meijer, 2002) and adaptive testing (Bradlow & Weiss, 2001;Bradlow, Weiss, & Cho, 1998). In person-fit analysis, the interest is mainly with identifying aberrant item-score vectors and inferring the cause of this aberrance, for example, for diagnostic reasons (e.g., did the respondent understand the test instruction?…”