# True sample standard deviation

Six Sigma – iSixSigma Forums Old Forums General True sample standard deviation

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• #40303

Schuette
Participant

From two technicians,ten samples of a population were collected and measurements were taken during a Gage R & R, with one replicate,total twenty results. The overall mean wwas collected.What is the true sample standard deviation? Isn’t just the square root of the sum of squares of individual standard deviations/ n-1?

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#124714

CT
Participant

In Excel program equation Functions you will see 4 different calculations for standard deviation the first is STDEV which estimates standard deviation based on a sample. STDEVA estimates standard deviation based on a sample, including logical values and test. Text and the logical value FALSE have the value 0; the logical value TRUE hase value 1. STDEVP Calculates standard deviation based on the entire Population given as agruments (ignores logical values and text); For GAGE R&R’s this is very useful because it takes in the whole population. STDEVPA Calculates standard deviation based on an entire population, including logical values and text. text and the logical value FALSE have the value 0; the Logical value TRUE has the value 1.
Remember standard deviation for sample and standard deviation for population are different, but will yield similar results as far as % of R&R. It is my recommendation that at least 3 measurements are taken for a sample of 10 pcs. Also use the Instaneous method for calculating R&R if the measurment process is automated (no operator interaction)
My 2 cents
CT

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#124715

BTDT
Participant

Jim:This depends on whether there is a systematic difference between the two operators. The usual Gauge R&R requires more than a single replicate, so I’m not sure how you are running the test.How are you getting the SS? Are you running the ANOVA Gauge R&R?BTDT

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