Standard Deviation
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 This topic has 5 replies, 5 voices, and was last updated 18 years, 5 months ago by Gabriel.

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August 5, 2003 at 6:35 am #32968
Why do we need Standard Deviation??
0August 5, 2003 at 7:10 am #88604Standard deviation is the measurment of your process variation.
0August 5, 2003 at 7:27 am #88605Why do we need to use standard deviation to find out the variation when their are other ways of doing it??
0August 5, 2003 at 1:42 pm #88616Why do we need to use any other method when there is standard deviation?
As long as it makes sense and can be interpreted, then there is nothing wrong with using another measure. SD is just very common and applies to all statistical testing.0August 5, 2003 at 2:04 pm #88621Actually its a good point. Standard deviation might not be a good measure if the data is not normally distributed. eg a span or range is a good measure when its a long tail distribution and Stability factor Q1/Q3 is a measure of variation when data is skewed. We need to see the distribution of data before deciding the rightr measure of variation. A normal probalbility plot would help.
Also Range is good measure of variability whre we have few data points ( like destructive testing). Range is also used to measure Repeatability error in measurement system analysis.
nayan0August 5, 2003 at 3:11 pm #88623
GabrielParticipant@Gabriel Include @Gabriel in your post and this person will
be notified via email.Usually, Range is just an intermediate step in the way to Standard Deviation. For example, in r&R studies or in proces capability studies from XbarR charts you get each R for each sugroup, then calculate the average range, and then calculate the standard deviation as Rbar/d2. Finally you use that standard deviation (not the range itself) to calculate the r&R or Cpk metrics.
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