The Stanford-Binet intelligence scale
When Lewis Ternan adapted Binet's intelligence test for the U.S., he coined the term intelligence quotient. This was defined as the result of deviding someone's mental age by their chronological age and multiplying it by 100.
Unfortunately, this scale has a number of problems. Most notable, is the fact that the scale breaks down completely at about 18 years of age. The tests are being extended to try and reduce this problem, but sucess has been variable.
Another notable problem with this scale is that it massively over-emphasizes crystaline intelligence. It does this to such a degree that for people with brain injuries, no significant change can show up on this test at all, even though the patient might show obvious signs of mental impairment.
Yet another problem with this scale is the over reliance on verbal and linguistic test components. This tends to mean that anyone for whom english is not their main language will probably get seriously marked down compared to a native english speaker.
A further problem with this scale (and most other scales share the same problem) is that for those whose intelligence is high enough to class them as a Genius or whose intelligence is low enough to class them as having Learning Difficulties, it proves very difficult to get a large enough sample to get the tests to provide accurate results. Therefore people at these two ends of the scale tend only to have very rough figures for their intelligence (although the testers sometimes fail to make this clear).
The Wechsler intelligence scale was designed to get around some of these problems, but it has only been partially sucessfull.