In statistics, sigma is a measure of the variation in a set of data, usually represented by the lowercase Greek letter sigma (σ) . It refers to the amount of variability in a given set of data, whether the data points are all clustered together or very spread out. The standard deviation is a common measure of sigma, which is the square root of the average of all the squared deviations from the mean. One standard deviation, or one sigma, plotted above or below the average value on a normal distribution curve, would define a region that includes 68 percent of all the data points. Two sigmas above or below would include about 95 percent of the data, and three sigmas would include 99.7 percent. In the context of Six Sigma, lowercase sigma is a measure of process capability, and the term Six Sigma means that the closest specification of a process or product is six standard deviations away from the mean of a normally distributed set of data.