Re: Sample size to test products used for 100 hours
Most likely you better test all. If I interpret your question correctly - you have put 25 units through a 100 hour test, and now you want to know how many need to be checked to see if they are satisfactory.
The thing is, even if you tested all 25, and they all came up satisfactory, at best all you could say is that you are 92% confident that at least 90% of the product are satisfactory after a 100 hour test.
Most organizations look for 95% confidence that at least 95% are good - and that takes 59 tests without failure to prove.
"A Passionate Statistician", ASQ CQE, ASQ Fellow