The number of students enrolled refers to the count of students studying in the reference period. Each student enrolled in the education programmes covered by the corresponding category is counted once and only once. National data collection systems permitting, the statistics reflect the number of students enrolled at the beginning of the school / academic year. Preferably, the end (or near-end) of the first month of the school / academic year is chosen (special arrangements are made for part-year students who may not start studies at the beginning of the school year). Exceptions to this may be required: at the pre-primary level of education a gradual inflow may exist and, therefore, an average over several counting dates may be preferable. At the tertiary level the enrolment of students may not be stable enough at the beginning of the academic year and therefore a count at a later point may be preferable. Adult education programmes cover the learning activity of those returning to education after having left initial (or "regular") education. Only programmes (of at least one semester) that are similar to regular education (in subject content or potential qualifications) are included. In some countries, there are, in addition, adult education that are not similar to regular education; these programmes are not included.
