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RE: [ai-geostats] Comparison of sample areas

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  • Colin Badenhorst
    Chris, How about using a T-test to test for similar sample means, or an F-test to test for similar sample variance? Regards, Colin ... From: C.J.Banks
    Message 1 of 5 , Feb 28, 2006
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      Chris,

      How about using a "T-test" to test for similar sample means, or an
      F-test to test for similar sample variance?

      Regards,
      Colin

      -----Original Message-----
      From: C.J.Banks [mailto:C.J.Banks@...]
      Sent: 28 February 2006 14:26
      To: ai-geostats
      Subject: [ai-geostats] Comparison of sample areas

      Dear All

      I have sampled three rectangular fields within a larger area and
      measured a variable of interest at a lot of points in each of these
      fields. Values were taken from all over each of the sampled fields but
      are not necessarily over a systematic grid. I am interested in testing
      whether the three sampled fields come from the same population. However,
      an ANOVA (or other similar tests) assumes that there is no correlation
      between values within each of the sampled fields, which isn't true
      because of the spatial nature. I'm sure that similar studies have been
      done and would appreciate any pointers to useful sources or appropriate
      statistical tests.

      For those who are interested the data are depths of snow on different
      floes in the same area of the Antarctic.

      Many thanks

      Chris



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    • Mach Nife
      Hi, I m hunting for a software (freeware/openSource if possible), that would help estimating the best possible semivariogram curve in a non-interactive way. As
      Message 2 of 5 , Feb 28, 2006
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        Hi,

        I'm hunting for a software (freeware/openSource if
        possible), that would help estimating the best
        possible semivariogram curve in a non-interactive way.
        As an example, ArcGis Geostatistical Analyst does a
        pretty good job at this when we accept the defaults.
        It does some automatic calculations for the parameters
        of the selected model. I've tried Gstat "Fit" method
        (in the command-line version), but the results aren't
        what I expected. What I need is a command line
        software or one that can be controlled by programming.

        Any ideas?
        machnife

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      • Mat (University Account)
        Hi Chris, It depends on the inference space you wish to work in. If your hypothesis is justing testing for a difference in the population means of the three
        Message 3 of 5 , Feb 28, 2006
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          Hi Chris,
          It depends on the inference space you wish to work in.

          If your hypothesis is justing testing for a difference in the population
          means of the three survey extents (fields),
          then any underlying autocorrelation in the population(s) is not really
          problematic
          - it is actually a bonus (i.e. you probably have better precision than you
          think).
          Geostatisticians are generally aware of this. They typically constrain
          inference about population means etc
          to the survey extent and are wary about extrapolation (after all it's
          generally considered unwise to think
          that one area has the same properties to other distant unsampled areas). By
          constraining inference to the survey extent, geostatistics (and other
          methods) can make use of autocorrelation to improve the precision of the
          parameter estimate.

          In fact, if you're able to take a random sample from _any_ population, then
          your classical (Neyman)
          confidence intervals and hypotheses about the population parameters will be
          valid.
          Fisher knew what he was doing :-).

          The problems occur when you try to increase your inference to include
          unsampled extents
          [this would generally be considered 'pseudo-replication' in ecology and
          earth sciences].
          If inference is extended beyond the range of the sample extent then by the
          sample can't be considered design-based (random), and autocorrelation has
          the opposite effect on your inference.
          i.e. its presence means that you have less information and fewer degree of
          freedom than classical tests assume.

          I am curious - this interaction between autocorrelation and inference space
          is never
          discussed by my ecology colleagues and rarely discussed in general and I am
          interested in finding out whether or not spatial scientists are well aware
          of this point? I thought the spatial inference scale in any spatial analysis
          should be something that is generally explicitly discussed since:
          (1) the scale of inference it is not always obvious when reading a paper,
          (2) it tends to have such profound effects.

          Perhaps I am being naïve in thinking this way? Feedback would be welcome.

          cheers,
          Mat


          -----Original Message-----
          From: C.J.Banks [mailto:C.J.Banks@...]
          Sent: Wednesday, 1 March 2006 3:26 a.m.
          To: ai-geostats
          Subject: [ai-geostats] Comparison of sample areas

          Dear All

          I have sampled three rectangular fields within a larger area and measured a
          variable of interest at a lot of points in each of these fields. Values were
          taken from all over each of the sampled fields but are not necessarily over
          a systematic grid. I am interested in testing whether the three sampled
          fields come from the same population. However, an ANOVA (or other similar
          tests) assumes that there is no correlation between values within each of
          the sampled fields, which isn't true because of the spatial nature. I'm sure
          that similar studies have been done and would appreciate any pointers to
          useful sources or appropriate statistical tests.

          For those who are interested the data are depths of snow on different floes
          in the same area of the Antarctic.

          Many thanks

          Chris
        • osei, samuel
          Hi Collins, I guess T and F tests are used for samples from TWO sampling campaigns. Please correct me if I m wrong. What Chris is referring to is more than 2
          Message 4 of 5 , Mar 1, 2006
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            Hi Collins,

            I guess T and F tests are used for samples from TWO sampling campaigns.
            Please correct me if I'm wrong. What Chris is referring to is more than
            2 sampled fields.



            Thanks


            Sam



            -----Original Message-----
            From: Colin Badenhorst [mailto:CBadenhorst@...]
            Sent: Tuesday, February 28, 2006 3:01 PM
            To: C.J.Banks; ai-geostats
            Subject: RE: [ai-geostats] Comparison of sample areas

            Chris,

            How about using a "T-test" to test for similar sample means, or an
            F-test to test for similar sample variance?

            Regards,
            Colin

            -----Original Message-----
            From: C.J.Banks [mailto:C.J.Banks@...]
            Sent: 28 February 2006 14:26
            To: ai-geostats
            Subject: [ai-geostats] Comparison of sample areas

            Dear All

            I have sampled three rectangular fields within a larger area and
            measured a variable of interest at a lot of points in each of these
            fields. Values were taken from all over each of the sampled fields but
            are not necessarily over a systematic grid. I am interested in testing
            whether the three sampled fields come from the same population. However,
            an ANOVA (or other similar tests) assumes that there is no correlation
            between values within each of the sampled fields, which isn't true
            because of the spatial nature. I'm sure that similar studies have been
            done and would appreciate any pointers to useful sources or appropriate
            statistical tests.

            For those who are interested the data are depths of snow on different
            floes in the same area of the Antarctic.

            Many thanks

            Chris



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

            This e-mail and its attachments, is confidential and is intended for the
            addressee(s) only. If you are not the intended recipient, disclosure,
            distribution or any action taken in reliance on it is prohibited and may
            be unlawful. Please note that any information expressed in this message
            or its attachments is not given or endorsed by Lisheen Mine unless
            otherwise indicated by an authorised representative independently of
            this message. Lisheen Mine does not accept responsibility for the
            contents of this message and although it has been scanned for viruses
            Lisheen Mine will not accept responsibility for any damage caused as a
            result of a virus being passed on.


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