How is a person's class determined in the 1984 world?

The class of
a person in the world is determined according to the class one is born
into and by ability as defined by the Party, according to the discussion in Part II Chapter IX.
There are three classes, the proles, the Outer Party, and the Inner Party. The proles make up
about 85% of the total population, the Outer Party about 13%, and the Inner Party about
2%.

If a person is born as a prole, they cannot enter any other class.
Extremely intelligent proles who appear to have the potential to make trouble are eliminated by
the Thought Police as a...