Hi Yuan,
The sample size recommended is a minimum of 32 to assure adequate statistical
robustness; unfortunately, many industry test programs utilize a smaller
number to reduce costs.
While a lognormal distribution can be fitted to the data, a better fit is
typically achieved with a Weibull distribution; the Weibull distribution is
used now almost exclusively.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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