Standard deviation is a measure of the spread of data in relation to the mean. It is the most common measure of the variability of a set of data. If the standard deviation is based on a sampling, it is referred to as “s.” If the entire data population is used, standard deviation is represented by the Greek letter sigma (s). The standard deviation (together with the mean) is used to measure the degree to which the product or process falls within specifications. The lower the standard deviation, the more likely the product or service falls within spec. When the standard deviation is calculated in relation to the mean of all the data points, the result is an overall standard deviation. When the standard deviation is calculated in relation to the means of subgroups, the result is a pooled standard deviation. Together with the mean, both overall and pooled standard deviations can help you determine your degree of control over the product or process.