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Hi Isobel, Thanks - that’s the answer I am
looking for. To illustrate why I am asking this question, and why I think the
definition is important : Assume a single structure spherical
correlogram model with Co = 0.25 C = 0.75 Co+C (Sill) = 1.0 Using Alwyn’s / Royle’s terminology: Nugget variance (Co) = 0.25 Nugget effect (epsilon) = 0.27 / 0.75 = 33%
(or 0.33) To quote Alwyn : “There is confusion in the
scientific literature concerning [nugget effect] which is also reflected in the
terminology used in software programs. The nugget variance is often
misleadingly referred to as nugget effect which is incorrect as, as we have
seen, is the ratio of Co/C. Similarly, the spatial variance is referred to as
the sill or sill variance which is also misleading in the authors opinion, as
the sill variance is Co+C which corresponds to the dispersion variance of the
date used to produce the semi-variogram” Mining Geostatistics
Course Book – Cardiff University, 1998 – this course book is taken
verbatim from Alwyn’s book. You are quite correct, it doesn’t
really matter what you call it as long as you understand its meaning. Regards, Colin From: Isobel Clark
[mailto:[EMAIL PROTECTED] Colin Alwyn is a graduate of the Whatever you call it, it represents the micro-scale variation which
cannot be predicted at the current scale of your sampling. Since we use a
semi-variogram, it is one-half of the variance between two samples at almost
exactly the same location. Geostatistics schools differ on what to do at exactly zero distance
(and software packages reflect this). Some schools (e.g. Stanford) insist that
nugget effect is due solely to sampling error and use the nugget semi-variance
at zero distance. Others say that the nugget effect is micro-scale (even
microscopic scale) and use it everywhere except at zero distance. At zero the
semi-variance is set to zero. A good question to ask your software vendor is
"what does your software do with the semi-variogram (or covarianc) at zero
distance". Reality is somewhere in between, but I do not know any software which
allows you to allocate some of the nugget effect at zero. Of course, you can
always get round this by adding a component with a very short range of
influence. Isobel
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