Research has demonstrated that people generally think both their knowledge and performance levels are greater than they are. Although several studies have suggested that knowledge and progress visualization offered by open learner modeling (OLM) technology might influence students' self-awareness in a positive way, insufficient evidence exists to show that this is the case. This paper examines the effects of open learner modeling and its extension with social comparison features, known as open social learner modeling (OSLM), on students' knowledge monitoring abilities. We report the results of two semester-long classroom studies, using subjects who were undergraduate and graduate students in Java Programming and Database Management courses at the University of Pittsburgh. During their studies, the students were able to use different versions of an online practice system equipped with both OLM and OSLM. The students' knowledge monitoring abilities were examined in two ways: through absolute and relative assessments. According to the results, although in both OLM and OSLM groups the students' absolute knowledge monitoring ability increased during the semester-long study, relative self-assessment ability (i.e., their ability to compare their own knowledge levels with the knowledge levels of their peers) only increased in the OSLM group. The authors also traced relationships between the students' academic achievement and their absolute and relative knowledge monitoring abilities.