Susan Durham <sdurham@BIOLOGY.USU.EDU> wrote
>Say you have a mechanical device that must withstand 20 lbs of pressure.
>You test 10 of these devices with a machine that is able to deliver up to
>400 lbs of pressure. None of the 10 devices fails. The final recorded
>pressure measurements for these 10 devices are close (but not exactly equal)
>to 400, with some variability but with a very small standard deviation. You
>think you need a confidence interval for the pressure at which the devices
>This question was posed to me by a fellow shuttle passenger on the way home
>from the airport. To an ecologist, I probably would say, "Wow, who needs
>statistics on data like that?!" But this is a medical device so
>expectations might well be different. I told him I'd ask around.
>My questions are:
>Even in a regulatory environment, is there any point in computing a
>confidence interval on data of this nature? If so, how might this
>confidence interval be computed?
>Is another form of assessment appropriate in this context (e.g., some
>probability of failure)?
>I had one QC/reliability course many, many years ago and am essentially
>clueless on this topic, so any suggestions would be welcomed.
I don't know much about this field at all, but it seems to me that the relevant statistic
is not a CI around the 400 pounds, but an estimate of what proportion would fail at 20 pounds.
Perhaps there are physical properties similar to a dose response curve that let you estimate this from the data you have, but I don't know.
Peter L. Flom, PhD
www DOT peterflomconsulting DOT com