Difference Between Standard Deviation Overall and Within?
- November 19, 2018 at 9:27 pm #208215
What is the difference between Standard Deviation (Overall) and (Within)? I’m currently using the formula for Overall, not sure of the formula for Within; or it’s purpose.November 20, 2018 at 1:38 am #208238
You can find the answer to this and most other questions by searching this site. It’s probably been discussed before. https://www.isixsigma.com/topic/difference-between-overall-stdev-and-within-stdev/December 1, 2018 at 10:00 am #208645
Basically overall is the standard deviation of all the data and the within represents the standard deviation of your subgroups. https://support.minitab.com/en-us/minitab/18/help-and-how-to/quality-and-process-improvement/capability-analysis/how-to/capability-sixpack/between-within-capability-sixpack/interpret-the-results/all-statistics-and-graphs/standard-deviations/December 3, 2018 at 10:28 am #208837
Answer is, standard deviation within is the short-term standard deviation from a sample, containing “normal” process variation. This standard deviation is used in Minitab control charts to calculate the control limits of a chart. Formula is, Average Moving Range / d2 for a sample size of 2. Standard deviation within is used to calculate Cpk.
Conversely, standard deviation overall is the long term standard deviation of a Population, which contains both “normal” and “special cause” variation. Formula for this is the base standard deviation formula for a population and is used to calculate Ppk.
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