The standard deviation is a term from statistics that describes the width of the probability curve, or how widely distributed the random values are. A small standard deviation means the values are tightly clustered around the mean and a large one means the values are widely scattered. For a normal distribution, 68% of the values fall within 1 standard deviation, 95% fall within 2 standard deviations and 99.7 fall within 3 standard deviations. This is commonly referred to as the 68–95–99.7 rule. For other distributions the values will be different. With a standard deviation of 0.33 and a mean of 0, 99.7% of the values will fall between -1 and 1. With a mean of 0.5 and a standard deviation of 0.166, you will get 99.7% of the values between 0 and 1 with the curve centered on 0.5.