What is IQ?
IQ is a measure of relative intelligence determined by a standardized test. The first intelligence test was created in 1905 by Alfred Binet and Théophile Simon to determine which French school children were too “slow” to benefit from regular instruction. Binet came up with the idea of mental age when he noticed that children are increasingly able to learn difficult concepts and perform difficult tasks as they get older. Most children reach the same level of complexity at about the same time, but some children are slower reaching those levels. A 6-year-old child who can do no more than a 3-year-old has a mental age of 3. Wilhelm Stern divided the mental age by the chronological age to get a “Mental Quotient.”
Mental Age/Chronological Age = Mental Quotient
A 6-year-old able to do only what a 3-year-old can do has a Mental Quotient of .5 or ½ (3 divided by 6). Lewis Terman later multiplied the Mental Quotient by 100 to remove the fraction and the Intelligence Quotient (IQ) was born!
Mental Age/Chronological Age X 100 = Intelligence Quotient
The 6-year-old with the Mental Quotient of ½ has an IQ of 50.
The majority of people have an IQ between 85 and 115.
(Adopted from about.com)
How does one qualify to join Mensa?
The qualification to becoming a Mensan is to have IQ over 98th percentile of world population. In general, the distribution of IQ in world population is assumed to have Gaussian/normal distribution with median lies at 100 and standard deviation of 15.
(Image adopted from Wikipedia.com)
There're numerous IQ tests available over the internet, following different standard with different scale of measurement. Lewis Terman, an American psychologist who coined the term Intelligence Quotient, considered the maximum cut-off of an IQ testing to be 140, while his colleague Leta Hollingworth put it at IQ of 180. The famous Raven's Advanced Progressive Matrices is a set of 48 questions, while some IQ testing available on the internet is a set of 39 questions with maximum IQ scale of 140, with Mensa requirement of having IQ 130 or above. In contrast, the IQ testing set used by Malaysian Mensa is a set of 36 questions with maximum IQ scale of 180. Taking top 2 percentile of world's population, the Mensa IQ requirement is set to be 148 or above.
Are Mensans geniuses?
Mensans are different from geniuses as geniuses stand at the proximity of the extreme (or outliers). Terman and Hollingworth considered genius to be having the cut-off IQ in an IQ test, namely 140 and 180. In modern times, genius is usually taken to be at the top 0.5 percentile of the world IQ distribution. So, don't mistaken Mensa as an organisation full of geniuses; not all of us are!