teachers: not what they used to be

[christian11 at mindspring.com]

>and the human capital of teachers may have declined in value relative to that of college graduates by as much as thirty percent, but the teacher-student ratio has more than doubled over the last half century in a wide array of developed countries. 

What is productivity in teaching if not student/teacher ratio (ie "output" of enculturated subjects)? If measured otherwise, how would you figure out whether declining productivity isn't the effect of higher student/teacher ratios rather than the other way around?

Christian