The Student Mobility Index is designed to be a highly sensitive statistic of classroom disruption. The idea behind the statistic is that any student entries or exits to and from a classroom during the year, over however many days, contribute to a disruption of instructional continuity and obstacle to institutional organization. This kind of student movement can have a negative impact on the moving student as well as all other students in the classroom. In practice, the degree of impact is affected by the percentage of moving students and the collective length of missed classroom time. Typically, the Student Mobility Index is calculated starting from the first day of classes for the school year. However, there appears to be a considerable amount of "trickling in" of students during the first week of classes. Any student entering a few days late during this first week gets counted as a "mobile" student and increases the mobility rate for that school. It may be assumed that it takes a week or so from the start of classes to settle in for the teacher and the students, and late entries during this first week, although potentially troublesome, may be of less relative impact on classroom continuity. Thus, adhering strictly to the definition of the mobility index as starting from the first day of classes, while reflecting real disturbance, may have a tendency to inflate the statistic and the inferred effect. This is especially true for Zone schools. These schools had a starting date one week earlier than regular schools for the 2005-06 school year. There are a number of alternative ways of measuring student mobility, each with its own valuable perspective. One measure is not more "correct" than another--merely different, and together provide a broader and deeper view of the concept of mobility. One such type of alternative measure is the Stability Index.