I do not know if it is a rule of thumb. But it also depends on the variability of your data values. I think it is quite subjective how much pairs you will use.
But you should also consider some more robust variogram estimators as correlogram or standardized variograms to see if there are no artificial structures or erratic structures caused by extreme values or outliers or even trends... There is a paper which is quite interesting: Srivastava, R.M.; Parker, H.M., 1989:Robust measures of spatial continuity. In: Armstrong, M., 1989 (editor):Geostatistics. Vol.I. Dordrecht, pp. 295-308. Regards, Ulrich On Wed, 1 May 2002, Arie Croitoru wrote: > Dear list members, > > Following Ruben Roa's email, I would like to add that I did not find any clear >criterion to what is the minimum number of data pairs that should be used in each lag >and this would certainly affect the variogram uncertainty. I know that a minimum of >30 data pairs is used frequently but my impression is that this is a "rule of thumb". >I guess this rule originated from the central limit theorem (?). I found that the >lack of a clear criterion might have a big effect on the estimation of the Variogram >especially in cases where the data points are non regular and in cases where a non >parametric variogram might be considered. > > I would gratefully appreciate any comments on this as well. > > Thanks, > > Arie. > -- __________________________________________________ Ulrich Leopold MSc. Department of Physical Geography Institute for Biodiversity and Ecosystem Dynamics Faculty of Science University of Amsterdam Nieuwe Achtergracht 166 NL-1018WV Amsterdam Phone: +31-(0)20-525-7456 (7451 Secretary) Fax: +31-(0)20-525-7431 Email: [EMAIL PROTECTED] http://www.frw.uva.nl/soil/Welcome.html Check us also out at: Netherlands Centre for Geo-ecological Research http://www.frw.uva.nl/icg -- * To post a message to the list, send it to [EMAIL PROTECTED] * As a general service to the users, please remember to post a summary of any useful responses to your questions. * To unsubscribe, send an email to [EMAIL PROTECTED] with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list * Support to the list is provided at http://www.ai-geostats.org
