IQ No Longer A Quotient
[T]here is really no longer an IQ, much less an IQ test. IQs are, literally, Intelligence Quotients, but the so-called IQ tests haven't yielded actual quotients for a few generations [...]
Originally, IQ was thought of as a ratio of Mental Age (MA) divided by Chronological Age (CA) and multiplied by 100. Mental age was the age at which a person was functioning intellectually according to the test. So if a person of any age scores as well as the average child of 8 on an intelligence test, then that person's MA = 8 years. For an 8-year-old, an MA of 8 yields an IQ of 100. At age 6, MA = 8 corresponds to an IQ of 133 (great performance), and at age 16, the IQ of 50 is not so good. The idea was clever, but it didn't work too well, because one year's growth in mental ability or height has very different meanings across the age range -- it corresponds to a great deal of growth from age 3 to 4, for example, but not so much from age 16 to 17. And what do you do with adults who are 25 or 40 or 80 years old? The whole notion of the ratio IQ falls apart.
So back in 1939, David Wechsler [...] got rid of the quotient and replaced it with standard scores, a terrific statistic. But he continued to call the overall scores IQs. [...]
The IQ as a ratio or quotient is long gone, and the IQ test label should be a thing of the past. Today's tests are referred to as cognitive ability tests, mental processing tests, or tests of multiple cognitive abilities by the professionals who develop the tests and by those who interpret them. But "IQ test" remains in the public's vernacular and is alive and well in the professional community as well. [...] [N]either [the] label ["IQ" or "IQ test"] is technically correct. But they do communicate. And they are much quicker to write and say than "Broad Cognitive Ability Composite" or "standardscore-yielding-multiple-cognitive-abilities test."
-- Alan S. Kaufman
from "IQ Testing 101 (Psych 101)"
Quoted on Fri Sep 21st, 2012