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From: Apache Wiki <wikidiffs@apache.org>
To: Apache Wiki <wikidiffs@apache.org>
Date: Tue, 27 Oct 2009 18:00:38 -0000
Message-ID: <20091027180038.6145.37729@eos.apache.org>
Subject: 
 =?utf-8?q?=5BCouchdb_Wiki=5D_Update_of_=22View=5FSnippets=22_by_MarcaJame?=
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Dear Wiki user,

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The "View_Snippets" page has been changed by MarcaJames.
The comment on this change is: Clarified why one would *want* to use the co=
mplicated algorithm..
http://wiki.apache.org/couchdb/View_Snippets?action=3Ddiff&rev1=3D28&rev2=
=3D29

--------------------------------------------------

  <<Anchor(summary_stats)>>
  =3D=3D Computing simple summary statistics (min,max,mean,standard deviati=
on)  =3D=3D
  =

- Implementation in {{{JavaScript}}} by MarcaJames.  Mistakes in coding are=
 my fault, algorithms are from others, as noted.  To the best of my knowled=
ge the algorithms are public domain, and my implementation freely available=
 to all (Perl Artistic License if you really need a license to consult)
+ This implementation of standard deviation is more complex than the above =
algorithm, called the "textbook one-pass algorithm" by Chan, Golub, and LeV=
eque.  While it is mathematically equivalent to the standard two-pass compu=
tation of standard deviation, it can be numerically unstable under certain =
conditions.  Specifically, if the square of the sums and  the sum of the sq=
uares terms are large, then they will be computed with some rounding error.=
  If the variance of the data set is small, then subtracting those two larg=
e numbers (which have been rounded off slightly) might wipe out the computa=
tion of the variance.  See http://www.jstor.org/stable/2683386, http://peop=
le.xiph.org/~tterribe/notes/homs.html, and the wikipedia description of Knu=
th's algorithm http://en.wikipedia.org/wiki/Algorithms_for_calculating_vari=
ance.
  =

- Here is some code I have developed to compute standard deviation.  I do i=
t two ways, both of which are different from jchris' github version (add li=
nk?).  In practice of course you wouldn't need both ways.  The view is spec=
ialized to my dataset, but the reduce function might be useful to others.
+ The below implementation in {{{JavaScript}}} by MarcaJames.  Any mistakes=
 in the js coding are my fault.  The algorithms are from others (all smarte=
r than I), as noted in the comments in the code.  To the best of my knowled=
ge the algorithms are public domain, and my implementation freely available=
 to all.  =

  =

- I've only ever tested it on futon, and have no idea what the "group" para=
meter does to the output.  Probably nothing!
+ Note that the view is specialized to my dataset, but the reduce function =
is written to be fairly generic.  I kept the view as is because I'm too laz=
y to write up a generic view, and also because when I wrote it I wasn't sur=
e one could use Date, Math, and Reg``Exp in Couch``DB Java``Script.  =

  =

  {{{
  // Map function