> I would like to know what is the declustering, why > we used this > method and how we proceed to decluster a set of > data? If your data is clustered spatially (in location) this may bias any histograms or probability plots which you draw and, therefore, any conclusions you make about what kind of distribution the values come from.
For example, in mining projects geologists tend to drill a lot more holes in the good bits than the bad bits. This means that a histogram contains a lot more samples than it should in the higher end. If you try to fit a model to such data, or use a transform or 'anamorphosis' it will not really reflect the values in the whole of the area. Backtransforms will be biassed like the original samples. Declustering is one way to get rid of the bias. There are various ways to decluster but the most common ones revolve around laying a grid of squares over your map area and either (a) selecting one sample per square or (b) averaging all the samples in each square. (b) is not very sensible given what we are trying to do with the data, but is very common (again) in mining. If you use (a) it is a good idea to choose which sample to 'keep' in the histogram at random. You may still use all of the clustered sampling for geostatistical analysis, of course. The semi-variogram and kriging techniques are not affected by clustering. In fact, one of the main reasons for inventing kriging was to make full use of every one of clustered and/or preferentially sited sampling. The term 'declustering' became popular around 1982/83 and is first used widely in the proceedings of the geostatistical congress held at Lake Tahoe in September 1983. Isobel Clark http://uk.geocities.com/drisobelclark __________________________________________________ Do You Yahoo!? Everything you'll ever need on one web page from News and Sport to Email and Music Charts http://uk.my.yahoo.com -- * 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