The term IQ stemmed from a German language term Intelligenz-Quotient, which was first introduced in 1912 by the German psychologist William Stern. His original idea was to develop a method to quantify intellectual capabilities of children. This method did not work for adult population and was replaced by another procedure introduced by David Wechsler in 1955. The Wechsler Adult Intelligence Scale or WAIS is now almost universally used in all IQ tests.
IQ test scores are used to characterize the level of intelligence of the test subject. They range from a minimum of 40 to a maximum of 175. A score between 40 and 45 represents a person who is severely challenged, while a person who scores between 160 and 175 is said to be extraordinarily genius. The test scores in between these two extremes have the following interpretations: 55-69 challenged, 70-84 below average, 85-114 average, 115-129 above average, 130-144 gifted and 145-159 genius.
It can be argued that an IQ test score does not necessarily represent the intellectual capabilities of a test subject. The main point to note here is that the skills required to solve the problems in a typical IQ test can be acquired through practice. Also, there is a strong component of familiarity with mathematical principles and the language of the test. A person who is well versed in both of these areas will do better than a person who does not have formal training or in-depth knowledge in these areas. This means that the IQ test score does not really measure intelligence but acquired proficiency in some areas.
Achievement tests are designed to measure the skills and knowledge acquired by an individual. The achievement test scores should not be confused with IQ test scores as the latter are supposed to measure the intellectual ability of the test person. However, studies have shown that there is some correlation between achievement test and IQ test scores.