greetings all!

I have a torque process that we are qualifying. i have 30 samples that have been tested and the distribution is non-normal.
the test is to establish a 99% confidence that failure would not occur until >5 turns.
we experienced no failure <=5 turns. but the distribution is skewed toward the limit.

can anyone suggest a model or method to determine the confidence level?
thanks,

If you can post the data (in an Excel spreadsheet is especially handy), it would greatly allow us to assist.

here's the data

thanks
Rob C

This sounds like a typical application of reliability engineering.

In reliability engineering, the Weibull Distribution is a commonly used instead of the normal distribution precisely because it can fit skewed, non-normal data.

There is lots of information on the web about Weibull analysis. A good website is http://www.weibull.com/basics/lifedata.htm They sell software, but they also have a lot of good information.

And I always recommend the NIST Engineering Statistics Handbook. They have quite a bit of info on reliability at http://www.itl.nist.gov/div898/handbook/apr/apr.htm, including discussion of Weibull analysis.


Tim F

P.S. if you post the data, there is a good chance someone will take a look at it.

I have attached the 'best fit' distribution data from your data. I used my favorite program - Distribution Analyzer from Taylor Enterprises (www.variation.com)

One more question.... Are the numbers listed the next higher number from when it failed? For example, if the failure is listed at "7", I am guessing that means it survived 6 turns, but failed some time before the 7th turn? As opposed to "7" meaning sometime between 6.5 turns and 7.5 turns.

I ran the numbers thru Minitab and found that the 3-Parameter Weibull, the 3-Parameter Lognormal, and the 3-Parameter Loglogistic all fit quite well.

The results are listed below (in a format that is unfortunately a little hard to read). The bottom line is that 1% of the parts are predicted to fail after 5.07, 5.31, and 5.22 turns for he three different models. Unfortunately, the lower confidences limits for the 1% failure time is 4.95 turns. So you couldn't be 95% confident that 99% would survive 5 turn.

At least, that is how I interpret the results....





Goodness-of-Fit

Anderson-Darling Correlation
Distribution (adj) Coefficient
3-Parameter Weibull 0.665 0.954
3-Parameter Lognormal 0.757 0.979
3-Parameter Loglogistic 0.754 0.986


Table of Percentiles

Standard 95% Normal CI
Distribution Percent Percentile Error Lower Upper
3-Parameter Weibull 1 5.06815 0.313992 4.95 5.72249
3-Parameter Lognormal 1 5.30569 0.620517 4.95 6.67256
3-Parameter Loglogistic 1 5.22087 0.287229 4.95 5.81530

3-Parameter Weibull 5 5.35177 0.250414 4.95 5.86578
3-Parameter Lognormal 5 5.57592 0.424557 4.95 6.47333
3-Parameter Loglogistic 5 5.54601 0.227759 5.11710 6.01087

3-Parameter Weibull 10 5.63980 0.240373 5.18781 6.13116
3-Parameter Lognormal 10 5.79599 0.319429 5.20254 6.45712
3-Parameter Loglogistic 10 5.80170 0.222758 5.38113 6.25515

3-Parameter Weibull 50 7.78835 0.485794 6.89211 8.80114
3-Parameter Lognormal 50 7.39866 0.458349 6.55271 8.35382
3-Parameter Loglogistic 50 7.38317 0.370192 6.69212 8.14559