Hi Chaosheng, It depends - if the systematic sample (SYS) grid is randomly positioned, then all (N) points in your sample extent have equal probability of selection (i.e. n/N), and therefore your estimate of the population mean is unbiased. However, all pairs of samples do not have equal probability of selection (for instance, neighbouring points in the sample extent cannot ever be selected), leading to difficulty in the estimation of the standard error (this, of course, is under the assumption that the population is [positively] spatially autocorrelated - if the population was independent, then the position of the samples does not really matter - i.e. a systematic sample is effectively the same as SRS).
The difficulty in determining inference for SYS using design-based methods has meant that model-based methods (e.g. geostatistics), where randomness arises from the stochastic spatial process described by the model, are sometimes used. Mind you, in ecology, the most common-method of treating SYS inference still tends to be to pretend that the data came from SRS .. this gives a conservative estimate, i.e. over-estimate, of the standard error). Regards, Mat --- Dr Matthew Pawley Post-Doctoral Fellow Leigh Marine Lab University of Auckland _____ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Zhang, Chaosheng Sent: Wednesday, 14 November 2007 12:41 a.m. To: [email protected] Subject: AI-GEOSTATS: Systematic grid sampling Dear all, In geochemical investigations, it seems that there is a growing trend to use a systematic sampling scheme based on a predefined grid system. Here is the question: Does such a systematic grid sampling belong to probability sampling or not? I have got a comment on this topic: "Here, there are no probability features of the geochemical variables because there is no chance experiment involved." Cheers, Chaosheng -------------------- Dr. Chaosheng Zhang Lecturer in GIS Department of Geography National University of Ireland, Galway IRELAND Tel: +353-91- 49 2375 Fax: +353-91- 49 5505 Email: [EMAIL PROTECTED] Web: http://www.nuigalway.ie/geography/gis
