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RE: AI-GEOSTATS: testing sample number

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  • Pierre Goovaerts
    Hi Mark, For each number of samples you wish to eliminate, you really need to repeat the sampling many times in order to account for sampling fluctuations in
    Message 1 of 5 , Mar 31, 2004
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      Hi Mark,

      For each number of samples you wish to eliminate,
      you really need to repeat the sampling many times in order
      to account for sampling fluctuations in the assessment
      of prediction performances.
      The procedure need to be automated and you won't avoid
      having to modify a program (i.e. Gslib kt3d whose source code
      is distributed freely) to implement the hundreds or thousands of
      run needed for a thorough analysis. Just as an illustration
      of the kind of fluctuations you can expect if you select
      randomly 100 subsets of the same size, look at the following
      paper that can be downloaded from my webpage:

      Saito, H. and P. Goovaerts. 2000. Geostatistical interpolation of
      positively skewed and censored data in a dioxin contaminated site.
      Environmental Science & Technology, vol.34, No.19: 4228-4235.

      Regards,

      Pierre

      <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

      Dr. Pierre Goovaerts
      President of PGeostat, LLC
      Chief Scientist with Biomedware Inc.
      710 Ridgemont Lane
      Ann Arbor, Michigan, 48103-1535, U.S.A.

      E-mail: goovaert@...
      Phone: (734) 668-9900
      Fax: (734) 668-7788
      http://alumni.engin.umich.edu/~goovaert/

      <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

      On Wed, 31 Mar 2004, Mark Dowdall wrote:

      >
      >
      > Hello
      >
      > This is a newbie question but I have been all over the Faqs and cannot
      > find an answer.
      >
      > Any help is appreciated and a summary of answers will be posted.
      >
      > I have a set of data that was taken over a particular area. I have
      > kriged and contoured and am happy with the results.
      >
      > But I need to demonstrate, within the bounds of the
      > study/analysis/assumptions, that the number of samples taken was
      > sufficient to describe the area.
      >
      > So my plan was to demonstrate that for a cetain number of samples the
      > variance in the estimates had reached a value that could not be reduced
      > efefctively by increasing sample number.
      >
      > And I thought this could be done by eliminating at random a point (so I
      > have x-1 data points), kriging the remainder with the chosen parameters
      > and checking the relevant parameters. (x is the number of actual
      > samples)
      >
      > Then eliminating two points at random (x-2), repeating and so on.
      > Eventually only 1 point being left.
      >
      > But.....if I eliminate the first point at random, does it matter which
      > point is eliminated? Or in the first instance (x-1 points) should I do
      > the process for all possible samples and take an average of the
      > estimation uncertainty?
      >
      > I am using GEOEAS and this could take a long time as all possible
      > permutations of for example x-20 could be quite large.
      >
      > I did download Explostat which has a feature that sems to do this
      > automatically but the manual is not great and the software is a little
      > impenetrable.
      >
      > If anyone can shed light on this I would be most grateful.
      >
      > Thanks in advance
      >
      > M.dowdall
      >
      > Ie.
      >
      >
      >
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