Student cases were weighted for sex and enrollment status (full-time, less than full-time) with a post-stratification weighting algorithm to minimize nonresponse bias (Little, 1993). This resulted in the creation of four cells for first-year students and another four cells for seniors. The weight for student cell h is given by:

Where r is the total number of institutional respondents of a given class, Ph is the 1998-99 IPEDS (for 2001 schools) or 1999-2000 IPEDS (for 2002-2003 schools) institutional proportion corresponding to cell h, and rh is the number of institutional respondents in cell h.

If sex or enrollment status could not be determined for a particular student, no weight was assigned to this student.In total, we were unable to assign weights to 3,738 out of 182,753 students (2.0 percent) due to missing values for sex or enrollment status.

If a student was not assigned a weight, s/he was not included in the calculation of her/his institution’s benchmarks. To enhance the stability of the weighting procedure, students were assigned a weight of unity if r_h was less than 5.

Reference
Little, R.J.A. (1993) "Post-Stratification: A Modeler's Perspective". Journal of the American Statistical Association. 88:1001-1012.