Lars Poulsen wrote: > > >I also cannot place any trust in figures that conclude that the units get > >even > >worse than the actual units tested from production. i.e. the worse production > >unit was 43 dB and the calculations come up with 43.41 dB. > > This is fundamental to statistical analysis. If the units do not come > off the production line with uniform characteristics, why should we > assume that the 10 samples we pick from a 1000-size lot include the > worst ? We can't ... we must assume that they actually follow a bell- > curve, and the computation of mean and standard deviation represents > our attempt at fitting the samples to the bell curve template in order > to determine the shape of that bell curve. Having selected a sample of > 8, and fitted a distribution that that, you may find that after drawing > 2 more samples and finding that they have BETTER data than any of the > previous samples, your standard deviation increased enough that the > projected worst case got worse, maybe enough to make you fail where > you passed before. > > Here's an example: > Assume legal limit 50 dB with 6dB margin. > > Samples 1-6 43 dB > samples 7-8 41 dB > samples 9-10 34 dB > > The first 8 samples yield a mean of 42.25, standard eviation 1.035 > and a 95% confidence interval of 41.53-43.29, i.e. 6.71 dB margin. > Adding the last 2 samples moves the mean to 40.60, but the standard > deviation jumps to 3.60, yielding a 95% confidence interval of > 38.37-44.20, i.e. the margin dropped to 5.80 dB. > > / Lars Poulsen [email protected] +1-805-562-3158 > OSICOM Technologies (Internet Business Unit) (formerly RNS) > 7402 Hollister Avenue Manager of Remote Access Engineering > Goleta, CA 93117 Internets designed while you wait
Thank you Lars. I've finally realized something key here that has bothered me since Cynthia raised the point. And Cynthia, let me distill what this has finally come to mean for me. I'm going to make it very simplistic especially with all the variables, but I'm going to use this as a rule of thumb from now on ... "If you're lucky, 10 will get you 6, but 6 will only give you 2." In other words, to actually have a good -6dB margin you have to shoot for -10dB. Shooting for a -6db margin as your limit may really only give you a -2dB margin. Greatly dependent upon many many factors. But at least it's put this whole discussion in perspective. I was expecting an entirely different response for no reason. Thanks everyone. ******************************************************* Doug McKean [email protected] ------------------------------------------------------- The comments and opinions stated herein are mine alone, and do not reflect those of my employer. ------------------------------------------------------- *******************************************************
