Hi Isabel If you go to www.isaaks.com and click on "Otherstuff", you will find an example where the block estimates are conditionally biased (rather severely), but the grade tonnage curves are right on the money. Perhaps this will help clear the confusion. Ed
-----Original Message----- From: Isobel Clark [mailto:[EMAIL PROTECTED] Sent: Monday, July 19, 2004 10:47 AM To: Edward Isaaks Cc: [EMAIL PROTECTED] Subject: [ai-geostats] Re: Kriging Small Blocks Ed I would differ from your explanation on one point. If you are merely declaring a mineral resource, i.e. mineral in the ground, then the conditional bias may not be relevant at the "pre feasibility" stage. However, as soon as you introduce any economic or technical parameters which entail selection, the conditional bias makes its appearance. In every project I have worked on, from pre-feasibility onwards, I have been asked for a grade/tonnage calculation - no matter how hand-waving it may be. The grade/tonnage curve will be affected by the conditional bias. By how much has to be assessed at the time. Most of Chapter 3 in Practical Geostatistics 1979 is devoted to working out what the (theoretical) global grade tonnage curve looks like when you adjust for the variance reduction. Even this will differ from the curve derived from the kriged estimates, no matter what size the block. The problem is even greater for environmental applications, especially toxic level risks. A 'global view' - i.e. a map - will not identify the true peaks because of the conditional bias. The fact that the overall average is unbiassed is irrelevant when trying to identify an area which is likely to be lethal. So, there is no contradiction. Conditional bias is unimportant (or irrelevant) until you apply some selection criterion. Yes, we agree. However, selection criteria can be relevant at very early stages of a project. It depends on your objective. Isobel http://uk.geocities.com/drisobelclark/practica.htm for free downloads of Practical Geostatistics 1979 PS: sorry I mis-spelled your name, I know it drives me nuts when people call me 'Clarke' ;-) ___________________________________________________________ALL-NEW Yahoo! Messenger - sooooo many all-new ways to express yourself http://uk.messenger.yahoo.com
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