EDUC categories pre- and post-1990

I am looking at educational attainment from 1965-2015 using the EDUC variable. I’d like to look at the following categories: Less than HS, HS Diploma or Equivalent, Some College No Degree, Associate’s degree, Bachelor’s Degree, Masters or Above.

Because of the variable change in 1990, I have a couple questions about comparability.

  1. Should I consider “3 years of college” equivalent to “Some college, no degree”?

  2. What are “12th grade, no diploma” and “12th grade, diploma unclear” comparable to in the categories I’ve listed above?

Thank you for your guidance!