At the risk of further extending this philosophical (if not etymological) discussion: in further defence of 'redundancy' I would point out that 'no longer needed' is not the only meaning of 'redundant', though admittedly it is the one that most often grabs the headlines! The meaning of 'redundant' in the context of employment is actually a relatively recent one and somewhat changed from the original meaning.
In a scientific context there's a second meaning of 'redundant', and in fact this one is much closer to the original one. In information theory the term 'redundant' applies to extra information added to a message being passed down a transmission line, in order to reduce corruption and loss of information, i.e. redundancy is absolutely needed to reduce the error rate. In a crystallographic context the purpose of redundancy, i.e. measurements over and above those strictly required to obtain a structure, is also obviously to reduce errors. 'Additional' here clearly does not necessarily imply 'not needed'. 'Redundant' comes from the Latin 're', meaning 'again', and 'unda', meaning 'wave', from which of course we get 'inundated' and 'undulator', so 'redundant' means literally 'coming in waves' or 'overflowing'. So we could say that redundancy is the process of being inundated by data from an undulator! As BR points out 'multiplicity' has long been used to indicate the number of equivalent reflexions generated by the point-group symmetry (so in PG222 h00, hk0 and hkl have respectively multiplicites of 1, 2 and 4 for non-zero hkl). I googled 'reflection multiplicity' and the top hit was http://pd.chem.ucl.ac.uk/pdnn/symm2/multj.htm . Suppose I want to express the following idea: "Redundancy is likely to be correlated with multiplicity". How do I express that unambiguously if 'redundancy' is redefined as 'multiplicity'? Cheers -- Ian On 18 January 2015 at 13:12, Bernhard Rupp <[email protected]> wrote: > In defense of redundancy: > > While the IUCr online dictionary is notably silent about multiplicity, the > term itself seems > already oversubscribed and used differently in various crystallographic > contexts. > > (i) Each general or special position in a crystal structure has a certain > multiplicity, defined by symmetry. > > (ii) General reflection multiplicity M is usually is defined by reflection > symmetry, and > when reflections are affected by special operations, the resulting > corresponding > lower multiplicity because they map onto themselves is accounted for in > the epsilon factor e. > > Btw a useful table of M and e is Iwasaki & Ito Acta Cryst. (1977). A33, > 227-229 > > (iii) In case of Laue patterns, overlap of higher order reflections is > also called Multiplicity afaik > (various Helliwell/Moffat et al papers explain this). > > So expanding the term multiplicity to include multiple instances of > measurements of the same reflections > - while admittedly avoiding the connotation of obsolescence - adds to its > promiscuous meaning, > where context becomes part of the definition.... > > I abstain from making any suggestions because in the past this has led to > interesting > but time-consuming philosophical discourse, proving in general the > multiplicity of my reflections > and positions redundant if not obsolete. > > Best, BR > > -----Original Message----- > From: CCP4 bulletin board [mailto:[email protected]] On Behalf Of Kay > Diederichs > Sent: Sonntag, 18. Januar 2015 09:28 > To: [email protected] > Subject: Re: [ccp4bb] Redundancy vs no of frames > > Dear Rohit Kumar, > > I prefer the term "multiplicity" instead of "redundancy" because the > latter has a connotation of "not really needed any more". > > The relation then is > > multiplicity = c * number_of_frames * oscillation_range > > where the constant c depends mainly on the space group. > > HTH, > > Kay > > On Sun, 18 Jan 2015 02:35:46 +0530, rohit kumar <[email protected]> > wrote: > > >Dear all, > > > >Can anyone tell me how to calculate number of frames from redundancy or > >vica versa > > > >Thank you > > >
