Dear Marcel Vallée, Thanks. I think the sampling density is good enough to reveal the spatial structure, and the extreme samples are located within the "hot spots". The problem is that the few values are still extremely high within the "hot spots". This may be what the "nugget effect" means.
I'm just wondering if these few extreme values should really be "discarded"/ "censored" or replaced. However, this could get some criticism as they may be "real". If it is hard to find the best way, I will have to "replace" all the extreme values with 99% or 98% percentiles. But I'm not sure if it is appropriate to do so. Cheers, Chaosheng Zhang ----- Original Message ----- From: "Marcel Vallée" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]>; "Chaosheng Zhang" <[EMAIL PROTECTED]> Sent: Thursday, December 13, 2001 10:40 PM Subject: Re: AI-GEOSTATS: Extreme values? > > Dear Chaosheng Zang > > The sampling interval is so wide that the high values could easily be related to "hot spots" of > higher grade contamination, i..e dumping areas for particular kinds of slags, mineralized > waste, etc. A property map might help. > > Have you contoured the data? If so, the sampling interval is so wide that real hot spots of > environmental significance might not show 2D distribution on such a wide sampling grid, > however. > > Regards > > Marcel Vallée, Eng,, Geo. > Geoconseil Marcel Vallée Inc. > 706 Routhier Ave > Québec, Québec G1X 3J9 > Canada > Tel: (1) 418 652 3497 > Fax: (1) 418 652 9148 > Email: [EMAIL PROTECTED] > > ============================================== > 13/12/01 08:01:48, Chaosheng Zhang <[EMAIL PROTECTED]> wrote: > > > > > Date: Thu, 13 Dec 2001 13:01:48 +0000 > > > > From: Chaosheng Zhang <[EMAIL PROTECTED]> > > Subject:AI-GEOSTATS: Extreme values? > > To: [EMAIL PROTECTED] > > > > > > > > Dear all, > > > > My question is: How to deal with the extreme/outlying values in a data set? > > > > I am dealing with heavy metal concentrations in soils from a mine area. The > > > > sample number is 223, and the samples are spatially evenly distributed with > > the sampling interval of 400 metres. There are several samples with > > extremely high values, which makes me feel uncomfortable. The percentiles of > > the dataset are listed as follows (in mg/kg): > > > > > > Zn Cu Pb Cd As > > Min 4 1 25 0.0 2 > > 5% 35 6 35 0.1 6 > > 10% 40 7 41 0.2 7 > > > > 25% 65 13 62 0.3 9 > > 50% 122 18 168 0.6 15 > > 75% 338 27 821 1.5 28 > > 90% 907 56 2799 2.8 58 > > > > 95% 1986 116 4490 4.2 80 > > 96% 2462 151 4698 4.9 82 > > 97% 3493 178 5413 6.2 91 > > 98% 4697 207 7609 8.3 111 > > > > 99% 6712 247 11750 12.4 184 > > Max 11473 1293 16305 48.5 1060 > > When doing geostatistical and statistical analyses, we need some confidence > > in dealing with the these very high extreme values which account for less > > > > than 2% of the total sample number. > > > > Any suggestions? > > > > Cheers, > > > > Chaosheng Zhang > > =================================== > > Dr. Chaosheng Zhang > > Department of Geography > > National University of Ireland > > Galway > > IRELAND > > > > Tel: +353-91-524411 ext. 2375 > > Fax: +353-91-525700 > > Email: [EMAIL PROTECTED] > > =================================== > > > > > -- > * 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 -- * 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
