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[ai-geostats] Comparison of sample areas

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  • C.J.Banks
    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
    Message 1 of 5 , Feb 28, 2006
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      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
    • 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 2 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 3 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 4 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 5 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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