In message <09c2d42ff0bfca4b829cdbe89b8f66ffe86...@g3w0637.americas.hpqcorp.net>, dated Wed, 27 Jun 2007, "Conway, Patrick R (Houston)" <[email protected]> writes:
> ><q> >Because ALL results of measurements of physical quantities on many >nominally-identical samples show a statistical distribution of values. > ><q> > > - where is the data to support this statement for EMI >measurements of all products? I think you are challenging a basic principle of physics. > ><q> >No, for the reason given above. A small number of samples doesn't >explore the whole distribution, especially since they are probably made >form one or a few batches of each component, so even those don't show a >real-world distribution of values. > ><q> > - not true. > For example, HALT/HASS testing. > In that discipline a small sample is used to predict the >whole distribution. 'Predict' - surely I need say no more. > (BTW- why do HALT/HASS engineers get so many samples, and >I have to manage with only one?? ;) ) 'so many' - yes, of course if you test a large number of samples, you can explore much more of the real distribution. > ><q> >But if you are going to make a million a month, you don't do just one >test; you apply the 80-80 rule and test however many the rule requires. >At that rate of production, you CAN afford the USD100k it costs. ><q> >- the 80-80 rule also is unfounded, since the assumption is that EMI >measurements are statistical and are well defined. You'd better take that up with CISPR/A, whose members would no doubt be interested in your views. > > I agree that it is highly likely that the measurements are >random, but with what distribution? > Perhaps it is more Chaotic than Gaussian? Or perhaps uniform? Again, I think you are challenging the way the Universe works. > >Here is an 80-80 counter-example: > What if a company only manufacture one a month, and it is for >the local military? > And suppose that only one out of 10 (better than 80-80 rule) has >unplanned susceptibility so that it malfunctions. Look at the conditions for applying the rule. > > And further suppose that a soldier gets injured because he/she >cannot see the enemy/fuel gauge/altitude. > The 80-80 rule, the statistics and the amount of margin don?t >really help that customer. > In that case, and many others, having a test margin and test >site measurement uncertainty are useless to the customer. I don't see the connection. For a 'one-a-month' production rate and a military customer, I wouldn't dream of applying the 80-80 rule. I don't see that because the 80-80 rule leads to unacceptable results if misapplied, that means that pass margin and considerations of measurement uncertainty are valueless. > >There are no easy answers to (the other)Brian's question. > I applaud Brian, and others like him, who ask the basic >questions in an attempt to understand the problem. > He is doing the background work to support and communicate the >reasons for compliance (limits + margin). > > >My contribution (if I have one) to Brian's quest is this: > It is given that electronic products, and companies that >manufacture them, have an absolute need for EM compatibility. > > I propose that the starting point of all discussion about margin >should be a self-assessment of the need for compatibility. > > (is the need life saving? Is it interoperability? Is it >avoidance of public humiliation? Is it &?) > > Once a company understands the drivers of compatibility, then >the quantity of margin needed (or not needed) will follow. I agree that the subject needs to be approached with insight, rather than an unthinking obedience. But regulatory requirements demand obedience, whether unthinking or not. > -- OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk There are benefits from being irrational - just ask the square root of 2. John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________
