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 the female students between twenty and twenty -one were enrolled in college, while in 2007 , 30% of the female students between twenty and twenty -one 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?