I was away for a while and when I came back I had some 60 messages in my mailbox (our folks here have been "busy" lately... :-) ). I've decided to read them all first before replying to issues I was involved with, so... Bill, this is it! Gene, you, again (as usual... :-) ), have done so superb a job on this one that this e-mail will go straight to my archives! ;-) Thanks, pal. You have so thoroughly and masterfully addressed this issue here that one could say: 'nuff said! :-) Nonetheless, there will be one last one to address, and my promised response will (finally) be on that one, ok, Bill? Marcus On Mon, 16 Jul 2001 19:02:27 Gene Mechtly wrote: >On Mon, 16 Jul 2001, Bill Potts wrote: >> ... >> One need go no further than Webster's New Universal Unabridged Dictionary. >> ... >Webster's definitions are insufficient for calculating the precision >and determining the accuracy of experimental data. Consider the following: > >Calculation of *precision* results from the analysis of *random errors* >in a set of measurements of the same quantity under circumstances which >are held as nearly constant as possible. Standard deviation (root mean >square deviation from the mean value) is the usual measures of the precision >of the set of observations. > >Accuracy is a determination or estimate of *systematic errors* (the >closeness (lack of deviation) of the mean value of a set of measurements >from a reference value established by better quality instruments, better >techniques, and usually by better qualified observers or laboratories. >Accuracy is ofter only an estimated value because a better reference value >is not readily available, or does not exist. > >A set of measurements can be of good precision but poor accuracy. >(e.g. tightly clustered measured values with small standard deviation, but >having a mean value widely in error compared with a more correct value) > >Another set of measurements can be of good accuracy but poor precision. >(e.g. widely scattered measured values but having a mean value with little >deviation from a more correct value) > >Try making a scatter graph of twenty hypothetical observed values which >illustrate good precision but poor accuracy; and a scatter graph of >twenty values which illustrate poor precision but good accuracy. > >This is an easy exercise for a person who understands the difference >between precision and accuracy! > >Gene. > > Get 250 color business cards for FREE! http://businesscards.lycos.com/vp/fastpath/
