To achieve the goals, we analyze the behavior over time of
students who first register and pay fees. We did that to
eliminate possible effects in case of changing the structure
of the institute. We first considered all students that have
entered into the students database between 2003 and 2007,
the total number is large, it is time consuming to analyze
the whole data set, so we took a sample and analyzed that.
We selected the same number of students from each time
slot over the whole entry period; the sample contains a total
of 2069 students.