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AI-GEOSTATS: Negative Variances reply

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  • Kevin Lowe Rfn
    Hi all I have listed all the replies and my original question below. Just to clarify my original question, I did not use any additive constant in this process.
    Message 1 of 1 , Nov 13, 2001
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      Hi all

      I have listed all the replies and my original question below.

      Just to clarify my original question, I did not use any additive constant in
      this process.

      It appears that it is a software problem. I was running the process on an
      older version of Datamine 5 and after talking to Datamine directly they told
      me that the problem has been fixed in the latest release of Datamine Studio
      V1.1.

      Kevin lowe

      -----Original Message-----
      From: Kevin Lowe Rfn [mailto:KLowe@...]
      Sent: Friday, November 02, 2001 1:55 PM
      To: 'ai-geostats@...'
      Subject: AI-GEOSTATS: Negative variances


      Hi all

      I have been experimenting with various kriging methods in estimating
      Witwatersrand gold grades (South Africa) using Datamine software. I found
      that using lognormal ordinary kriging has resulted in negative variances in
      some blocks.

      My question is why does this happen and what is the significance of it?

      Kevin Lowe
      e-mail KLowe@...

      ****************************************************************************
      ******************
      Kevin

      There is something badly wrong with your software.

      With ordinary kriging, it is not possible to get
      negative variances.

      With lognormal kriging, it is not possible to get
      negative varianse or negative estimates unless you are
      using a large additive constant.

      Isobel Clark
      http://uk.geocities.com/drisobelclark

      ****************************************************************************
      *********************

      Dear Kevin,

      Please contact your Datamine agent around your area. I found that bug
      programming was fixed on lst release of Datamine Studio v.1.1 build 1136.

      regards,

      iman permadi
      ****************************************************************************
      *********************

      Hi Kevin

      I see you are using the list. I assume that you are referring to the values
      that you obtained from the back calculation. This indicates that the beta
      factor (additive constant) is invalid/ tooo big. This also implies that you
      are not seeing a three parameter log normal distribution(which can be
      deduced from your distributions descriptive statistics in log space,
      skewness =0 and kurtosis =3) and may be Compound Log Normal or LNGIG.

      You could try non parametric kriging (Indicator or Probability kriging).

      Cheers

      Mark

      ****************************************************************************
      ***********************

      Hi Kevin

      We too are Datamine users and I was wondering whether you had any resolution
      to this issue or indeed have you uncovered a bug?

      I note that Isobel replied that there must be a serious fault in the
      software - have you pursued this?

      Regards

      Sharron Sylvester
      Supervising Geologist
      HATCH ASSOCIATES


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