Ed, no the fact that you don't, can't or won't estimate the precision doesn't change anything (only as you say it becomes a poorly designed experiment). A measurement has a standard deviation regardless of whether you possess an estimate of its value or not. The exact true value of the standard deviation can never be known, just as the true value of any physical quantity can never be known, even after measuring it umpteen times! The measurements are only estimates of the true value, sampled from the error distribution of the true value.
The experimental estimate of the standard deviation is called the 'standard uncertainty' (indeed I remember when it was called the 'estimated standard deviation' or e.s.d.), again sampled from the error distribution of the SD. Sometimes I see in the literature the term 'estimated standard uncertainty' but this is a term that does not appear in any literature on statistics (it seems to be peculiar to protein crystallography literature!). Also it would then be the 'estimated estimated standard deviation' which is one more level of estimation that you need (an estimate of an estimate is still an estimate - it just has a bigger uncertainty than the previous estimate!). See http://physics.nist.gov/cgi-bin/cuu/Info/Constants/definitions.html for the terminology approved by NIST. Cheers -- Ian On 13 March 2013 20:36, Ed Pozharski <[email protected]> wrote: > Ian, > > On Wed, 2013-03-13 at 19:46 +0000, Ian Tickle wrote: > > So I don't see there's a question of wilfully choosing to ignore. or > > not sampling certain factors: if the experiment is properly calibrated > > to get the SD estimate you can't ignore it. > > > > So perhaps I can explain better by using the same example of protein > concentration measurement. It is certainly true that only taking one > dilution is "poor design". (Although in crystallization practice it may > not matter given that it is not imperative to have a protein exactly at > 10 mg/ml, 9.7 will do). If I don't bother including pipetting precision > in my error estimate either by direct experiment or by using > manufacturer's declaration I am willfully ignoring this source of error. > That would be wrong. > > But what if I only have one measurement worth of sample? And pipetting > precision cannot be calibrated (I know it can be so this is hypothetical > - say pipettor was stolen and company that made it is out of business, > their offices burned down by raging mob). Is the pipetting error now > systematic because experimental situation (not design) prevents it from > being sampled or estimated? > > I actually like the immutable error type better for my own purposes, but > I am trying to see whether some argument might stand that allows some > error that can be sampled to be called inaccuracy nonetheless. > > Cheers and thanks, > > Ed. > > > -- > I don't know why the sacrifice thing didn't work. > Science behind it seemed so solid. > Julian, King of Lemurs > >
