The proportion of female students enrolled in higher education programs has increased over the past years. This is partly shown by the fact that in 2000 only 10% of female students aged 20‑21 were enrolled in college, while in 2007 30% of female students aged 20‑21 were enrolled in college. To evaluate the argument above, it would be most useful to compare 2000 with 2007 with regard to which of the following characteristics?