An intelligence quotient, or IQ, is a score derived from one of several different standardized tests designed to assess intelligence.
The term “IQ” comes from the German Intelligenz-Quotient. When modern IQ tests are constructed the median score is set to 100 and a standard deviation to 15.
Today almost all IQ tests adhere to the assignment of 15 IQ points to each standard deviation but this has not been the case historically.
Approximately 95% of the population have scores within two standard deviations of the mean.
If one SD is 15 points, then 95% of the population are within a range of 70 to 130.