How was the amount of Native American blood deceided on in the 1930 census?

Hi,
The blood was sampled by a mechanical reading device, and was discarded in 1933, for lack of accuracy. It was divided in about 4 stages.
Here is a link for more information:

http://genealogy.about.com/od/native_american/a/census_rolls_5.htm

I hope this will help.