3 sigma and 1 sigma values
3 sigma and 1 sigma values
(OP)
I have what seems to be a simple statistics question, but has stumped everyone in my office (including me)...
I did a tolerance stack analysis on an assembly by doing a RSS (root sum squares) analysis. From what I understand, this results in a 3-sigma value. Is there a way to convert this number to a 1-sigma value? If so, how is it accomplished?
I did a tolerance stack analysis on an assembly by doing a RSS (root sum squares) analysis. From what I understand, this results in a 3-sigma value. Is there a way to convert this number to a 1-sigma value? If so, how is it accomplished?





RE: 3 sigma and 1 sigma values
TTFN
RE: 3 sigma and 1 sigma values
TTFN
RE: 3 sigma and 1 sigma values
tol 1: +-.005
tol 2: +-.010
tol 3: +-.010
RSS = sqrt((.005^2)+(.010^2)+(.010^2)) = .015
All of the information I could find regarding RSS analyses for tolerance stack applications concluded that the result (.015 in this example) is a 3-sigma value.
This being the case, can I simply divide by 3 to get a one sigma value? Using the same logic, would I multiply the result by 2 to get a six sigma value?
RE: 3 sigma and 1 sigma values
In a standard normal distribution about 68.3% of the data lie within 1 standard deviation of the mean. 95.5% or the data lie within 2 standard deviations of the mean. 99.73% of the data lie with 3 standard deviations of the mean.
Regards
Dave
RE: 3 sigma and 1 sigma values
NozzleTwister
Houston, Texas
RE: 3 sigma and 1 sigma values
Cheers
Greg Locock
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
RE: 3 sigma and 1 sigma values
In a previous life we had a tolerance stackup question. The Quality Department referred to Juran's Quality Manual. It indicates that RSS applies ONLY if all of your individual tolerances are normal distributions with Cpk of >= 1.33. Otherwise your tolerance stackup is arithmetic summation. You probably ought to verify this "fact" because I'm going on memory.
The summation, in reality, isn't very practical because the total tolerance could be very unreasonable. So where's the balance? Dunno. We just muddled through using so-called "engineering judgement".
TygerDawg