More

IQ Test Scores: The Basics of IQ Score Interpretation

IQ stands for intelligence quotient. Supposedly, it is a score that tells one how “bright” a person is compared to other people. The average IQ is by definition 100; scores above 100 indicate a higher than average IQ and scores below 100 indicate a lower that average IQ. Theoretically, scores can range any amount below or above 100, but in practice they do not meaningfully go much below 50 or above 150.

Half of the population have IQ’s of between 90 and 110, while 25% have higher IQ’s and 25% have lower IQ’s:

Descriptive Classifications of Intelligence Quotients
IQ	Description	% of Population
130+	Very superior	2.2%
120-129	Superior	6.7%
110-119	High average	16.1%
90-109	Average	50%
80-89	Low average	16.1%
70-79	Borderline	6.7%
Below 70	Extremely low	2.2%

Apparently, the IQ gives a good indication of the occupational group that a person will end up in, though not of course the specific occupation. In their book, Know Your Child’s IQ, Glen Wilson and Diana Grylls outline occupations typical of various IQ levels:

140	Top Civil Servants; Professors and Research Scientists.
130	Physicians and Surgeons; Lawyers; Engineers (Civil and Mechanical)
120	School Teachers; Pharmacists; Accountants; Nurses; Stenographers; Managers.
110	Foremen; Clerks; Telephone Operators; Salesmen; Policemen; Electricians.
100+	Machine Operators; Shopkeepers; Butchers; Welders; Sheet Metal Workers.
100-	Warehousemen; Carpenters; Cooks and Bakers; Small Farmers; Truck and Van Drivers.
90	Laborers; Gardeners; Upholsterers; Farmhands; Miners; Factory Packers and Sorters.

IQ Expressed in Percentiles

IQ is often expressed in percentiles, which is not the same as percentage scores, and a common reason for the misunderstanding of IQ test scores. Percentage refers to the number of items which a child answers correctly compared to the total number of items presented. If a child answers 25 questions correctly on a 50 question test he would earn a percentage score of 50. If he answers 40 questions on the same test his percentage score would be 80. Percentile, however, refers to the number of other test takers’ scores that an individual’s score equals or exceeds. If a child answered 25 questions and did better than 50% of the children taking the test he would score at the 50th percentile. However, if he answered 40 questions on the 50 item test and everyone else answered more than he did, he would fall at a very low percentile — even though he answered 80% of the questions correctly.

On most standardized tests, an IQ of 100 is at the 50th percentile. Most of our IQ tests are standardized with a mean score of 100 and a standard deviation of 15. What that means is that the following IQ scores will be roughly equivalent to the following percentiles:

IQ	Percentile
65	01
70	02
75	05
80	09
85	16
90	25
95	37
100	50
105	63
110	75
115	84
120	91
125	95
130	98
135	99

An IQ of 120 therefore implies that the testee is brighter than about 91% of the population, while 130 puts a person ahead of 98% of people. A person with an IQ of 80 is brighter than only 9% of people, and only a few score less than 60.