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GEOSTATS: Logtransform/Logratio transform

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  • sean beehan
    I have two problems which I hope someone may help me with. As a background, one of the aims of my PhD is to estimate the distribution of physical and chemical
    Message 1 of 1 , Jul 29, 1998
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      I have two problems which I hope someone may help me with. As a
      background, one of the aims of my PhD is to estimate the distribution of
      physical and chemical soil attributes across a valley (approx 10K km2).

      1. Transforming negatively skewed compositional data
      I am analysing soil at six depths for a variety of physical and chemical
      attributes. I have three separate compositional data sets. All values are
      positive. Within some depths of each set are outliers which skew the data
      to the left and some outliers which skew the data to the right. Sampling
      and laboratory results have been checked and the values appear to be
      correct. What is the most consistent way to treat each set? I understood
      that logtransform and logratio transform was not appropriate for negatively
      skewed data but am unsure why and what the ramifications are? Is it in the
      back transform?

      2. Bimodal distribution
      Within one of the compositional data sets (CEC) I have two attributes (Mg%
      & Ca%) which at one depth (20-30cm)are skewed bimodally to the left.
      Plotting the values against the co-ordinates the peaks fall roughly
      east/west. The remaining two attributes are positively skewed. My
      question is similar - how to treat the set consistently before kriging?

      Thanks in advance

      Alison Todd
      Dept Ag Chem & Soil Science
      University of Sydney
      Australia


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