The actual value of 1 sigma will depend on the test being used. For IQ tests, 1 sigma = 15 points. The 68-95-99.7 rule tells us that 68% of the population is between -1 and +1 sigma from the average; 95% is between -2 and +2 sigma from the average; and 99.7% is between -3 and +3 sigma from the average.