In a normal curve, 68% of the numbers are within 1 standard deviation and 95% are within 2 standard deviations.
A normal curve (also called a bell curve) occurs frequently. For example, people’s height, weight, and IQs all plot to a normal curve. Many random events such as flipping a coin or measurement errors also plot to a normal curve.
Beyond normal curves, in any curve, at least 75% of data are within 2 standard deviations.
The top 5% are two standard deviations above the mean.
The standard deviation of each section of the SAT test is very roughly 95.
There is not a single standard deviation for all IQ tests, but 15 may be most common.
But the very next day, his first Monday up here as a visitor, brought another standard deviation from the routine—and that was one of the lectures Dr. Krokowski gave in the dining hall every two weeks before the entire German-speaking, nonmoribund, adult population of the Berg-hot.