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1785[ai-geostats] RE: F and T-test for samples drawn from the same p

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  • Isobel Clark
    Dec 3, 2004
      Colin

      You need to bear in mind that statistical tests such
      as t and F are only testing a very simple hypothesis -
      they do not test whether the samples are from the same
      population.

      The F test is to check whether the standard deviations
      differ. If the ore is from the same genesis, it is
      likely that the variability will be constant and your
      F test will not be significant.

      The t test is against the hypothesis that the average
      values are the same. That is, one population has a
      higher average grade than the other. You can have the
      same variability around the mean, but have a zone
      where the minerals tend to concentrate at a higher
      average.

      Even if both tests are not significant, this does not
      'prove' that the two populations are the same. You
      could have two sets of data with the same mean and
      standard deviation and completely different shapes,
      for example.

      To include the spatial element, you could try a cross
      validation approach where one set of samples is the
      'actual' values and you try to estimate those from the
      other set. This will show up consistent differences in
      average between the two as well as differences in
      variability.

      Strictly, all of the above requires a Normal
      distribution but with your not-too-skewed data and
      thousands of samples, the Central Limit Theorem should
      take care of those problems.

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