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Hi Tim that feels right, but are the mean and variance uncorrelated? in mathematical analysis the mean always seems to be related to the first derivative of something or other, and the variance to the second. Not only that, but in most parametric distributions, the mean and variance incorporate the same parameter, with the normal distribution being the exception? Or do I just need more coffee this morning...? Andy On Fri, 22 Mar 2019 at 09:04, Cole, Tim <[log in to unmask]> wrote: > > Sorry to come late to the party. > > It¹s worth stating that accuracy relates to the mean of a sample of values > (in that bias is the difference between the sample mean and the true > mean), whereas precision relates to the sample variance (or its inverse). > Hence the two are uncorrelated by definition. > > Best wishes, > Tim Cole > > [log in to unmask] <mailto:[log in to unmask]> Phone +44(0)20 7905 > 2666 Fax +44(0)20 7905 2381 > Population, Policy and Practice Programme > UCL Great Ormond Street Institute of Child Health, London WC1N 1EH, UK > > ------------------------------ > > > > >Date: Thu, 21 Mar 2019 09:56:44 +0000 > >From: Allan Reese <[log in to unmask]> > >Subject: Precision & accuracy (MY conclusion) > > > >Thanks to all for comments. Martin Bland suggests, "The standard > >statistical usage, as I understand it, is that precision refers to how > >close repeated observations are to one another, and accuracy how far > >they are from the true value." That, and similar comments, correspond to > >the definitions in Kendall & Buckland (Dictionary of Statistical Terms, > >4th ed), though I would use "reliability" for the former. As K&D > >comment, the "precision" of an estimator varies with the square root of > >the number of observations, but it's not quite the sense here. > > > >John Whittington sent me a great hint off-list, "Modern technology is > >such that it is only too easy to produce such a device which displays > >umptreen more DPs than is sensible in relation to the accuracy of the > >measurement." That's what happen here. The device (digital calliper) has > >a standard LED display that shows 2dp, but it was flimsily constructed > >and flexed when applied to an object. There is no reason to believe the > >measurements are biased, especially as the "accuracy" is quoted as +/-. > >To my mind, it means a reading of, say 3.45 could mean anything from > >3.25 to 3.65, so I have doubts even rounding to 0dp! > > > >Fortunately, the model I use is made of metal, has a quoted precision of > >0.02 (and a 2dp display), so I'm happy rounding to 1dp. > > > >There is clearly much imprecision, and confusion, in the use of the word > >"precision" with regard to measurement (recording), calculation > >(numerical analysis), and general use in the language - Collins > >Dictionary was no help at all! > > > >Allan > > > > You may leave the list at any time by sending the command > > SIGNOFF allstat > > to [log in to unmask], leaving the subject line blank. You may leave the list at any time by sending the command SIGNOFF allstat to [log in to unmask], leaving the subject line blank.