- Dear group,

I read an interesting article about constructing confidence intervals for

variograms in order to assess the validity of grouping several sample sites

together to estimate one composite variogram. (Kabrick et al. 1997.

spatial patterns of carbon and texture .....Soil Science Society of America

Journal 61:541-548.

I would like to do a similar thing: to see if it's valid to group

variograms calculated from data from each of several smaller, non

contiguous plots in a forest into one composite variogram. Basically, the

Kabrick article points out that assessing confidence interval overlap is

not really a statistical test of similarity of two regression lines, so

they give a z test to further support a conclusion of similarity or

dissimilarity; I was, however, confused by this.

I was a little unclear on their approach and was wondering if anybody had

any more straightforward approach to comparing variograms. It seems like

one could estimate confidence intervals by calculating the standard errors

of all of the (xi-xi+h)^2/2n's, and multiply this std error by t (1.96 for

large n) to define the limits of the CI. Is this overly simplistic? I'm

sort of an intermediate geostatistics (and classical statistics for that

matter) user.

Thanks for any advice.

Andrew

alister@...

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DO NOT SEND Subscribe/Unsubscribe requests to the list! - Andrew and others,

We are interested in this issue as well in the context of

subsampling from a GIS coverage to conduct semivariogram

analysis. My colleagues are concerned by the idea that we

would be throwing away data and want to compare semivariograms

from the whole set with those estimated from samples (I'm

suggesting sampling with a mixed design of regular and more

closely spaced nearest neighbors).

In the past, I've assumed that the best way to test the

adequacy of the semivariogram is to do cross-validation --

krig based on the estimated semivariogram(s) leaving out

one point at a time, sum the squared errors (predicted Z - actual)

and compare with the kriging variance estimate (which is based

on the semivariogram and the spacing of data). This ensures

that the distances that will actually be used most are given

the correct weight. For example, if the data are all fairly

closely spaced, it doesn't matter if the semivariogram is way

off at large distances. If comparing semivariograms is the

issue, then I'd just be inclined to pick the one that does

a better job of producing accurate kriging variances.

I'd be interested in dissenting or confirming opinions, since

we are dealing with the same controversy.

Yetta

At 11:16 AM 9/30/98 -0400, you wrote:>Dear group,

------------------------------------------------------

>

>I read an interesting article about constructing confidence intervals for

>variograms in order to assess the validity of grouping several sample sites

>together to estimate one composite variogram. (Kabrick et al. 1997.

>spatial patterns of carbon and texture .....Soil Science Society of America

>Journal 61:541-548.

>

>I would like to do a similar thing: to see if it's valid to group

>variograms calculated from data from each of several smaller, non

>contiguous plots in a forest into one composite variogram. Basically, the

>Kabrick article points out that assessing confidence interval overlap is

>not really a statistical test of similarity of two regression lines, so

>they give a z test to further support a conclusion of similarity or

>dissimilarity; I was, however, confused by this.

>

>I was a little unclear on their approach and was wondering if anybody had

>any more straightforward approach to comparing variograms. It seems like

>one could estimate confidence intervals by calculating the standard errors

>of all of the (xi-xi+h)^2/2n's, and multiply this std error by t (1.96 for

>large n) to define the limits of the CI. Is this overly simplistic? I'm

>sort of an intermediate geostatistics (and classical statistics for that

>matter) user.

>

>Thanks for any advice.

>Andrew

>alister@...

>

>--

>*To post a message to the list, send it to ai-geostats@....

>*As a general service to list users, please remember to post a summary

>of any useful responses to your questions.

>*To unsubscribe, send email to majordomo@... with no subject and

>"unsubscribe ai-geostats" in the message body.

>DO NOT SEND Subscribe/Unsubscribe requests to the list!

>

Yetta Jager

Environmental Sciences Division

Oak Ridge National Laboratory

P.O. Box 2008, MS 6036

Oak Ridge, TN 37831-6036

OFFICE: 423/574-8143

FAX: 423/576-8543

Work email: jagerhi@...

Home email: hjager@...

WEBpage: http://www.esd.ornl.gov/~zij/

-----------------------------------------------------

"One man's mean is another man's Poisson" J.W. Haeffner

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