In IQs, a ratio IQ is the intelligent quotient of a person calculated as the ratio of the person's "mental age" (intellectual age of the person, based on a standardized test) to "chronological age" (actual age):

The ratio IQ is basically synonymous with the term Terman IQ, although the latter would generally be considered as an IQ determined by American psychologist Lewis Terman or by one of his tests, such as the Stanford-Benet.

Issues
The ratio IQ formula method was originally designed to test for normalcy and in particular retardation in school children, whereby as such the formula become inaccurate outside of this range and also inaccurate when used with those of adulthood age range (16+).

The issues associate with this formula become most noticeable when parents calculate extreme high-end IQs for their 4-9 year old children, by virtue of the fact that the all one needs is to find a test geared towards eighteen-year-olds and get the child to pass it with scores on par with the average age range of that student, and thus incorrectly conclude via calculate that the child has a 200-range near or above IQ, which would incorrectly but the child above that of the Cox-Buzan IQ estimate of Isaac Newton's IQ of 193, which is the benchmark for a realistic IQ estimate in the genius ceiling range.

All-in-all, IQs calculated via the age ratio method, generally have only a limited applicability, loosely accurate in the 40-170 range, outside of which values calculated become essentially meaningless.

The "deviation IQ", an intelligence quotient determined by comparing a person's test score with other examinees of the same age, was in fact developed, from 1914 to 1949 by David Wechlser, owing to the inadequacies of the applicability of the ratio method when used with adults. [1]

References
1. Colangelo, Nicholas and Davis, Gary A. (1991). Handbook of Gifted Education (pg. 92). Allyn and Bacon.

External links
● DeLacey, Margaret. (2004). “Ratio and Deviation Test Scores”, Tagdpx.org.