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Re: Statistics - Calculating Standard Deviation

Subject: Re: Statistics - Calculating Standard Deviation
From: "Dimitre Novatchev" <dnovatchev@xxxxxxxxx>
Date: Fri, 13 Jun 2003 19:17:33 +0200
calculating standard deviation
"Andrew Welch" <AWelch@xxxxxxxxxxxxxxx> wrote in message
> > The performance is the thing that is worrying me most.  Ideally the
> > target processor is MSXML 4.0, but that is open to negotiation...
> Well using saxon 7.x (use the latest) and exslt/math you could use the
following simple stylesheet.  Im just wondering how much > of this can be
done using straight xslt 2 now... Is there a square root function? I had a
quick look but didn?t see anything.

The solution I posted earlier today runs OK without any modifications in
XSLT 2.0 (Saxon 7.5):


> <?xml version="1.0"?>
> <xsl:stylesheet version="1.0"
>   xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
>   xmlns:exsl="http://exslt.org/math">
> <xsl:variable name="mean" select="sum(/root/node) div count(/root/node)"/>
> <xsl:variable name="diffs">
>   <root>
>     <xsl:for-each select="/root/node">
>       <node squaredDiff="{exsl:power($mean - .,2)}">

Why is this necessary? Probably multiplying a number with itself in pure
XSLT will not be slower?

>          <xsl:copy-of select="."/>
>       </node>
>     </xsl:for-each>
>   </root>
> </xsl:variable>
> <xsl:variable name="mean.Of.Sum.Of.Diffs">
>   <xsl:for-each select="$diffs">
>     <xsl:value-of select="sum(/root/node/@squaredDiff) div (count
>   </xsl:for-each>
> </xsl:variable>
> <xsl:template match="/">
>   standard deviation: <xsl:value-of
> </xsl:template>
> </xsl:stylesheet>

This solution will use 2 * N units of memory, which may be limiting its
applicability especially when processing long node-sets.
It may require from three to five traversals of a node-set with the length N
of the initial node-set (one each for sum() and count())

An advantage (in efficiency) is that it does not require any recursion.

However, I guess it would be much more efficient if sequences were
used/built instead of node-sets.


Dimitre Novatchev.
http://fxsl.sourceforge.net/ -- the home of FXSL

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list

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