[ai-geostats] Re: Who is J. W. Merks???
Jan Merks is an expert in sampling theory and works as an independent consultant out of Vancouver, Canada. He has a web site which I don't have to hand, where all of these opinions are repeated and amplified. Jan first starting publishing anti-geostatistics articles in 1991 or 1992 and the article Geostatistics or Voodoo Statistics appeared in every mining publication from the Engineering and Mining Journal to the Northern Miner newspaper. He republishes every so often and had one a few years ago in the Mining Journal on April 1st. The articles start with a quotation from Tolstoy to the effect that even the most intelligent of people can turn a blind eye to facts that don't fit their own world view. It is ironic that he does not realise this quotation is apropriate to his own world view too. His basic premise is that geostatistics is a con job foisted on an unsuspecting industry by consultants trying to rip them off for large sums of money. He supports this view by pointing out that the semi-variogram is divided by the number of pairs of samples (N) and not by N-1 when every statistician knows that variances are divided by N-1 not N. The point missed here is that variances are divided by N-1 because we estimate the population mean. Semi-variograms are not divided by N-1 because we assume the population mean (difference) to be zeto and do not estimate it. His second point is that kriging with (say) k samples should have k-1 degrees of freedom. This is not true becuase the variance/covariance or semi-variogram terms used in the kriging system are based on the total number of pairs used in the construction of the graph. I once asked Noel Cressie about this and he said that the degrees of freedom in the kriging system would be n(n-1) where n is the total number of samples in the data set. Back in 1992, I invited Dr Merks to come down to a course I was giving in Reno to put his point of view and debate it with myself and the students and staff at University of Nevada-Reno. I still have his letter on file. It basically says, I don't see the point you aren't going to listen anyway. Before you ask, the only reason I did this was because his articles referred to only two geostatistical publications: Michel David's Mining Geostatistics and my Practical Geostatistics (1979). He also couldn't spell my name right and I wanted to give him the opportunity to change that. It was several years before an editor pointed out to him that there is no 'e' on the end of Isobel Clark. Isobel http://uk.geocities.com/drisobelclark/practica.htm * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] Re: Who is J. W. Merks???
Thanks, Isobel, for the explanation. Some people never get it, the difference between design-based and model-based methods. From the other side it is also true that there are geostatisticians, even famous ones, who keep arguing that design- based methods (methods that essentially assume independent observations) are not applicable to spatial problems, even when spatial random sampling has been used. A useful reference for design-based methods is: DE GRUIJTER, J. J. C. J. F. TER BRAAK (1990), Model-Free Estimation from Spatial Samples: A Reappraisal of Classical Sampling Theory. Mathematical Geology 22, pp. 407-415. -- Edzer * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] Re: Who is J. W. Merks???
Hi, I think this man just wants to get more attention by making some noise. Geostatistics is the great contribution of geoscientists outside the mainstream of statistics to the science. I also ever met a reviewer. The guy never agree that parameters should estimated from correlated data. At the beginning of the development of kriging, I guess Professor G. Matheron must met lots of challenges. As Professor Journel pointed out, geostatistics does not pursue recognition of mainstream mathematics, it pursues recognition from practioners. It is great because it is useful. If just checking geostatistics purely from the view of conventional mathematics, there are lots of places to attack. But our understanding of geostatistics also should not stop on the conventional geostatistics. Geostatistics is also evolving. The recent proposed multi-point geostatistics is quite innovative. In addition, Markov chains are also evolving toward a new non-kriging geostatistics. Weidong Li University of Wisconsin Department of Geography 550 North Park Street Madison, WI 53706-1404 426 Science Hall E-mail: [EMAIL PROTECTED] -- - Original Message - From: Edzer J. Pebesma [EMAIL PROTECTED] Date: Saturday, April 30, 2005 6:51 am Subject: Re: [ai-geostats] Re: Who is J. W. Merks??? Thanks, Isobel, for the explanation. Some people never get it, the difference between design-based and model-based methods. From the other side it is also true that there are geostatisticians, even famous ones, who keep arguing that design- based methods (methods that essentially assume independent observations) are not applicable to spatial problems, even when spatial random sampling has been used. A useful reference for design-based methods is: DE GRUIJTER, J. J. C. J. F. TER BRAAK (1990), Model-Free Estimation from Spatial Samples: A Reappraisal of Classical Sampling Theory. Mathematical Geology 22, pp. 407-415. -- Edzer * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] Re: Who is J. W. Merks???
Dear List: "AI-GEOSTATS is mainly a mailing list which acts as a forum for the dissemination and discussion of all aspects of spatial statistics" "Tolerance is one essential key of the success of the mailing list." During these years I learned too much with the high level discussions in this list. I will learn more and more. Dogmas aren´t geostatistical tools. During all humankind history, the unanimity were employed like argument to many abuses. I like when somebody said I was wrong because I might be wrong. Think about this and excuse my poor english. Sincerely, Thanks for all. * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
Re: [ai-geostats] Re: Who is J. W. Merks???
AI Geostats Subscribers, Isobel Clark(e), It seems that Merks is his own worst enemy. As a matter of interest, when one is a member of the geostatistics church - ie member of a known geostatistical union, it is easy to criticise Merks for both his opacity and his criticism. Where there is smoke there is fire. The moment you believe your own hypothesis you are a dead duck scientifically. What geostatisticians never come to grips with in economic geology, where Merks consults for some of the senior players, is that your 'nugget effect' and the 'geologist's' nugget effect are homenyms for unreliability; but they do not mean the same thing. Practicing miners know that fudge factors, mine call factors, cutting, are used in the majority of metal mines in the world, simply because geostatistical forecasts are only accurate about 50% of the time (Aus IMM Special Publ. on Best Practices). It's garbage in, garbage out; bad data in bad data out. Danie Krige, however, actually published a summary paper in 2001 in SAJG in which he stated that the scientific mathematical underpinnings of geostatistics were well understood and not a lot of future research would be required in this field. He stopped just short of calling geostatistical theory a paradigm. I take issue with a field in which hundreds of data points, thousands of data points and perhaps tens of thousands of data points do not yield a trustworthy mean 50% of the time. I, like Merks believe that 30 or 31 values should tell the tale: semi-variograms excepted. That means I accept them. I do not know whether Merks does or not. I find the semivariogram model intellectually satisfying. I do not accept averages or cut weighted averages for a block of ore, a stope in a mine, or a global average - for an annual report. This is where the voodoo operates. I have found ore bodies to be chaotic. Sampling with a drill is very reliable in order to define the geometry, volume, and specific gravity of a deposit. Drill an oriented pattern and a few angle holes the opposite direction to test your imagination and the tonnage of a deposit can be estimated with a fair degree of accuracy. To ask a drill to sample for grade is another story. Every metal distribution is diferent. Metal concentrations depend upon the distribution of nucleation sites or structural preparation of the host rock. Assays are rarely reproducible. Twinned holes are rarely reproducible. Give me the mean and standard deviation and standard error of the mean of a global ore reserve from a typical 'nuggety' ore deposite, and I can reproduces those statistics with a similar drill program of twinning holes. However, only 10 or 11% of the twinned holes will be similar; a few will be identical twins, a few will be fraternal and the rest of the twins will be just family. Some may deny their paternity/maternity completely. This mystery has got a common name in geostatistics but I do not remember what Krige calls it today. For the 50% of deposits that behave as predicted, bravo for (GEO)statistics. However, what about the rest? We sweep them under the rug and rely on cutting or the mine call factor and wait a year to adjust the numbers with a fudge factor. In the Old Lead Belt of Missouri, the 'dilution' factor was 10%. When the miners moved to the new lead belt they applied 10% dilution, but it was not enough and 15% 'dilution' was called for. ...And the new lead belt was known for massive galena. At Elmwood, Tennessee Zinc held off production for a time while they attempted to reconcile assay from underground mine opening samples to nearby drill holes; it didn't work. They just went ahead an mined it and worked backward from concentrate grade and tailings to determine the feed grade. Ore deposit grades are chaotic and, if you will, the sample we take is too small at least 50% of the time. We then split the sample to laboratory size, and the laboratory then splits it to a smaller size 50 - 30 - 15 grams, depending on the cost of the assay (the budget). Bigger aliquots cost more. 50% of the time these samples are too small. Shall I say 50% of the deposits need custom sampleing. And the worst part of it is that we do not seem to account for that potentially fatal flaw at the beginning of the exercise. Colomak, NWT had a 25% shortfall from low grade ore. We do not think it significant to test for what the deposit is telling us before we begin the SOP of QCQA. [Standard Operating Procedure of Quality Control and Quality Assurance]. I say GIGO [Garbage in; garbage out.]. Isobel Clark calls it a systematic error - an error in data due to the method of collecting. Isobel also explains, near the back of her book, that if there is a systematic error, geostatistical methods are useless. At least that's my reading of it. For 50% of gold deposits, there is an error in data that results in lower ounces produced than estimated and higher cost per ounce than budgeted. This gives
[ai-geostats] Re: Who is J. W. Merks???
Hello people Thank you for your swift responses, especially on the weekend. This turned out to be a long reply, so feel free to read the next paragraph and skip on to the last one. I think we should be fair to Jan Merks. He got a bee in his bonnet over an issue which is less than well explained in the bulk of geostatistics text books, especially 13 years ago. He tried discussing it with some geostatisticians and got the usual how dare you criticise us reaction from the mainstream. If you don't believe me, get hold of the Engineering Mining Journal and read Danie Krige's response to Merk's article. I have never met Jan Merks and issued my invitation to discussion in the hope that we could learn something from one another. It was turned down with no opening for any continuation of debate, even by email. As a practising mining engineer who has to earn a living valuing mineral resources, I use what works in reality and follow (as much as I am capable of) new theories and practice as they become proven. As Fran says, every orebody is different and uncertainty is part of our way of life. The best we can do is minimise it and quantify what is left, if we can. There are many weak patches in geostatistical theory. However, we are not going to fix them by roaming through Amazon and writing hostile reviews of every book we can find on the topic. Or by ignoring opportunities for discussion and debate. Merks has a very powerful position in Canada, as he sits on the National and Government committees which determine standards for sampling design and such like. He is also, judging by the bulk of his own work, a very intelligent and persuasive communicator. And he is not alone. Read, for example, Philip and Watson's paper in Mathematical Geology in the mid-1980s. It took me several years to figure out why their antagonism to geostatistics was so strong. Finally, after a conversation with a land surveyor in South Africa, I realised that they did not know that we cannot see the surfaces we are mapping -- unlike map makers. Geostatistics is not a panacea and (in my opinion) is not a suitable method for automated mapping. Not till we patch the weak places, like semi-variogram modelling, conditional bias and handling non-Normal data. However, until someone comes along with something that is as easy to understand, test and apply I am sticking with it. I never wrote a response to Merk's article. How can you take a guy's criticism seriously when he can't bother to spell your name right? Isobel (with an o) Clark (without an e) http://geoecosse.bizland.com * By using the ai-geostats mailing list you agree to follow its rules ( see http://www.ai-geostats.org/help_ai-geostats.htm ) * To unsubscribe to ai-geostats, send the following in the subject or in the body (plain text format) of an email message to [EMAIL PROTECTED] Signoff ai-geostats
