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Re: AI-GEOSTATS: How to use CV

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  • Syed Abdul Rahman Shibli
    I always thought the CV would be more useful as a means to compare two different distributions with dissimilar means. Two CVs measured at two different
    Message 1 of 3 , Jan 22, 2004
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      I always thought the CV would be more useful as a means to compare two
      different distributions with dissimilar means. Two CVs measured at two
      different locations in the farm will indicate the relative dispersion
      between the two locations, since both standard deviations would be
      normalized by their mean values. Reading your post it's almost as if
      you're thinking of using it to determine if a distribution of values is
      skewed, for which the coefficient of skewness might be a better
      measure.

      Regards,

      Syed

      On 22 Jan 2004, at 06:30, Ian Hunt [HQP] wrote:

      > Dear List,
      >
      > I would appreciate a general discussion/recommendations on the use of
      > the coefficient of variation (CV) in terms of sampling reliability
      > firstly and secondly in the choice of when not to do ordinary kriging.
      >
      > The first case concerns the agricultural soil sampling situation where
      > a farm is sampled and then the CV (on the raw data) is quoted locally
      > as some form of reliability or risk figure. Does this make sense ? It
      > seems that a farm with a high CV correlates to high variability in the
      > farm and therefor high risk i.t.o crop estimates and crop modeling.
      >
      > In the second case some people have mentioned that a range of a CV of
      > less than 0.5 is good for ordinary kriging and for bigger than 0.5 it
      > is necessary to transform the data to logs or something else. What is
      > a good practical number to use or what is a good tool to use for the
      > choice of normality (including the use of the histogram and
      > probability plot)
      >
      > Thanks in advance
      >
      > Ian Hunt
      >
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    • Isobel Clark
      Hi Ian The CV is useful if you are measuring things which can only take positive values and tend to be positively skewed - i.e. lumped towards the zero. In
      Message 2 of 3 , Jan 23, 2004
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        Hi Ian

        The CV is useful if you are measuring things which can
        only take positive values and tend to be 'positively'
        skewed - i.e. lumped towards the zero. In that case,
        the standard deviation itself is not a measure of
        spread but something rather more complex. The CV is a
        simple measure of skewness under these conditions.

        In general:

        CV<1/3 might not be skewed

        CV around 1, moderately skewed

        CV much above 1, highly skewed

        The CV is directly related to the standard deviation
        of the logarithms in a lognormal situation.

        Does this help?

        Isobel Clark
        http://geoecosse.bizland.com/whatsnew.htm



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