The challenge for Goddard earlier in the century was to make sure people understood the difference between "feebleminded" and "moron." The challenge for most of the second half of the century was to expand the categories and attempt predictions on what individuals' lives would be like.
In the 1950s, the education system adopted a classification system based on ability to learn (Kirk, Karnes, & Kirk, 1955).
"Educable" children could learn simple academic skills but not progress above fourth grade level.
"Trainable" children could learn to care for their daily needs but very few academic skills.
"Custodial" children were considered untrainable and therefore in need of long term care.
The terms "trainable" and "educable" are still in use today. By 1971, the President's Committee on Mental Retardation indicated that "educable" children could achieve sixth grade level academic skills, instead of only fourth grade, by their late teens.