The sample standard deviation s is the square root of the sum of squared deviations from the mean divided by n minus 1; sigma is the true process value it estimates. Quality work separates short-term sigma, estimated within subgroups from ranges or moving ranges, from long-term or overall sigma computed from all the data, and the gap between them measures between-subgroup variation. Variances add for independent sources; standard deviations do not.
Within-lot standard deviation is 0.8, overall is 1.9. Most of the spread is between lots.
Add variances, not standard deviations: a gauge sigma of 3 and a process sigma of 4 combine to 5, not 7.
Also heard as
- sigma
- s
- std dev
- SD
Related terms
Normal distribution
Symmetric bell-shaped distribution set by a mean and a standard deviation, the default model for measurement data.
Cpk
Process capability index that accounts for centering: distance from the mean to the nearest spec limit in units of 3 sigma.
Ppk
Process performance index: a Cpk-style figure computed with the overall standard deviation, capturing long-term variation.
Variance components
Breakdown of total variation into the shares contributed by each source, such as lot, wafer, site and measurement.