The second empirical issue in need of attention relates to
the denominator of Eq. (1), the per period income received
by student loan debtors. The very important point is that
research so far has used very aggregate proxies of incomes,
such as that received by graduates on average. Ziderman
(2003), for example, in an analysis of the repayment burdens
associated with the SLF, compared debt servicing
obligations to the earnings of graduates derived from the
use of Thai earnings functions.