Re: Maximum Likelihood Question
Herman Rubin [EMAIL PROTECTED] wrote in message 9vqoln$[EMAIL PROTECTED]">news:9vqoln$[EMAIL PROTECTED]... Maximum likelihood is ASYMPTOTICALLY optimal in LARGE samples. It may not be good for small samples; it pays to look at how the actual likelihood function behaves. The fit is always going to improve with more parameters. This may be the trouble in the actual problem being attempted, but there are other possibilities, besides the potential for having programmed things incorrectly. One such trouble might be that the parameters are constrained and that the maximum-likelihood estimates given such constraints are falling on the edge of the allowed region .. then the usual asymptotics don't apply. David Jones = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
What does these mean in statistical sense??
I have 2 random variables (X and Y). The covariance,c was found equal to 20.2006 and their correlation coefficient,p was 0.0245. From the statistical book, if their c=0, means that X and Y are uncorrelated i.e p=0. However, in my case, c is quite large but p is extremely small...So, what justification could I said with this kind of data?? Thanks.. CCC = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
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Re: claculate L2 for Venus
Brad Guth wrote: My URL has much to do with the discovery of GUTH Venus http://geocities.com/bradguth One of my questions has to deal with a manned mission, which may need to utilize the orbit station L2, as an orbit situated so as to sustain life onboard the spacecraft for several months to perhaps years. If the space craft were to be represented by upwards of 1000 tons mass, what would the calculated L2 become? The same calculation at 500 tons and perhaps 100 tons. I've asked this of many NASA types and they exploded. Please don't explode on my account. A good lead to an astro/gravity-calculator may do just fine. *BANG!* Just kidding. I presume that by the orbit station L2 you mean the (unstable) second Lagrange point of the Sol/Venus pair? The location of this point is not affected to first order by placing a mass there. The only effect would be a second-order effect due to the perturbation, due to the station's mass, of the orbit of Venus [and, if you want to get truly silly, of the position of the Sun.] As in this case order of magnitude is essentially the ratio of the mass of the station to that of the Sun [not of Venus, which is the object being moved...] you can see that the empty location of L2 would continue to be valid in the presence of any object we could put there, probably to within a micron. That said, I feel I must add something on the subject of the NASA Venus images that you think show artifacts. I do not know if you are familiar with either marquetry or gemmology. If you have any experience with the first, you will probably know about the American red gum tree (_Fluidambar_styrax_ - what a beautiful name!). Its veneer is much sought after by marquetarians, because a slice cutting through both heartwood and sapwood often contains a detailed desert scene, with cirrus clouds in the sky and sand dunes on the ground. The dunes are often even silhouetted against the skyline. There are also sedimentary rocks that are used for jewellery which, when sliced, regularly show landscapes in which the eye recognizes many details. Again, I have seen a poster with an entire alphabet made up of photographs of details from butterflies' wings; and while *some* resemblances between such markings and other creatures are presumably evolutionarily advantageous in that the resemblance is to something a predator will avoid, this hardly explains the sometimes near-perfect human skull seen on the Death's-Head Moth. [You will have seen it on the cover of most paperback editions of The Silence of the Lambs. On some of these it's retouched, so that the skull itself is made up of several ghostly female bodies; but the basic image is fairly accurate.] What I'm getting at is that people tend to underestimate the ability of naturally-arising phenomena to mimic other things, without intelligent intervention. You should also be aware that these photographs were not taken using light, but by radar. If I remember correctly, they were not even taken as images, but as a linear scan pattern, assembled into an image by a computer on Earth. The bright lines are NOT differently-colored regions, or uniformly higher (or lower) than those around them; they are cliffs between terraces. If you imagine that the model is on a table top, made of dark clay, and lit from the side, it will be easier to interpret. Or think of a landscape seen at sunset. -Robert Dawson = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
Re: What does these mean in statistical sense??
Chia C Chong wrote: I have 2 random variables (X and Y). The covariance,c was found equal to 20.2006 and their correlation coefficient,p was 0.0245. From the statistical book, if their c=0, means that X and Y are uncorrelated i.e p=0. However, in my case, c is quite large but p is extremely small...So, what justification could I said with this kind of data?? It measn the variances are large. If s_A is the standard deviation of A, then p_XY = c_XY/(s_X*s_Y) So for your data, s_X*s_Y = 824.5. This is why we use p, it's re-scaled so that the variances are 1, so we can compare correlations of variables with different variances. In this case, p looks very close to 0. Bob -- Bob O'Hara Metapopulation Research Group Division of Population Biology Department of Ecology and Systematics PO Box 17 (Arkadiankatu 7) FIN-00014 University of Helsinki Finland tel: +358 9 191 28779 mobile: +358 50 599 0540 (Yes, I have finally joined 21st Century Finland) fax: +358 9 191 28701email: [EMAIL PROTECTED] To induce catatonia, visit http://www.helsinki.fi/science/metapop/ It is being said of a certain poet, that though he tortures the English language, he has still never yet succeeded in forcing it to reveal his meaning - Beachcomber = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
Re: What does these mean in statistical sense??
This is a multi-part message in MIME format. --D8246F46A01791942B12542B Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit try doing a scattergram of your two variables. It should look much more like a cloud than a line. Anon. wrote: Chia C Chong wrote: I have 2 random variables (X and Y). The covariance,c was found equal to 20.2006 and their correlation coefficient,p was 0.0245. From the statistical book, if their c=0, means that X and Y are uncorrelated i.e p=0. However, in my case, c is quite large but p is extremely small...So, what justification could I said with this kind of data?? It measn the variances are large. If s_A is the standard deviation of A, then p_XY = c_XY/(s_X*s_Y) So for your data, s_X*s_Y = 824.5. This is why we use p, it's re-scaled so that the variances are 1, so we can compare correlations of variables with different variances. In this case, p looks very close to 0. Bob -- Bob O'Hara Metapopulation Research Group Division of Population Biology Department of Ecology and Systematics PO Box 17 (Arkadiankatu 7) FIN-00014 University of Helsinki Finland tel: +358 9 191 28779 mobile: +358 50 599 0540 (Yes, I have finally joined 21st Century Finland) fax: +358 9 191 28701email: [EMAIL PROTECTED] To induce catatonia, visit http://www.helsinki.fi/science/metapop/ It is being said of a certain poet, that though he tortures the English language, he has still never yet succeeded in forcing it to reveal his meaning - Beachcomber --D8246F46A01791942B12542B Content-Type: text/x-vcard; charset=us-ascii; name=Arthur.Kendall.vcf Content-Transfer-Encoding: 7bit Content-Description: Card for Art Kendall Content-Disposition: attachment; filename=Arthur.Kendall.vcf begin:vcard n:Kendall;Art tel;work:301-864-5570 x-mozilla-html:FALSE adr:;; version:2.1 email;internet:[EMAIL PROTECTED] fn:Art Kendall end:vcard --D8246F46A01791942B12542B-- = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
Re: What does these mean in statistical sense??
Thanks for all the helpful suggestions... Cheers, CCC Art Kendall [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... try doing a scattergram of your two variables. It should look much more like a cloud than a line. Anon. wrote: Chia C Chong wrote: I have 2 random variables (X and Y). The covariance,c was found equal to 20.2006 and their correlation coefficient,p was 0.0245. From the statistical book, if their c=0, means that X and Y are uncorrelated i.e p=0. However, in my case, c is quite large but p is extremely small...So, what justification could I said with this kind of data?? It measn the variances are large. If s_A is the standard deviation of A, then p_XY = c_XY/(s_X*s_Y) So for your data, s_X*s_Y = 824.5. This is why we use p, it's re-scaled so that the variances are 1, so we can compare correlations of variables with different variances. In this case, p looks very close to 0. Bob -- Bob O'Hara Metapopulation Research Group Division of Population Biology Department of Ecology and Systematics PO Box 17 (Arkadiankatu 7) FIN-00014 University of Helsinki Finland tel: +358 9 191 28779 mobile: +358 50 599 0540 (Yes, I have finally joined 21st Century Finland) fax: +358 9 191 28701email: [EMAIL PROTECTED] To induce catatonia, visit http://www.helsinki.fi/science/metapop/ It is being said of a certain poet, that though he tortures the English language, he has still never yet succeeded in forcing it to reveal his meaning - Beachcomber = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
Re: Standardizing evaluation scores
A classic problem of 'norming' or 'standardizing' the scale and the preceptors. Can you find a couple students who fall near the bottom and tops of the scale? Preferably ones whose final rankings are not 'permanent record'? then you would have each preceptor use these two students as 'baseline' indicators of what a 2 means, and what an 8 means. then have each person do the regular ranking of students, using these as your indicators. It might be possible for the attendant group of preceptors to agree on the ranking of a pair of students, in each specialty or area. then use these for ranking within that specialty. Failing this kind of development for mutual agreement, you might be able to describe a 2 or 3 rating, and a 7 or 8 rating, in such a way that generalized agreement would be obtained, and each grade would be set in comparison to this descriptive scale. This is essentially what the Baldrige Criteria does, for industrial/ educational/ health care operations. Of course, if it's grades we are discussing, it is entirely likely that virtually nobody gets grades in certain ranges, such as the equivalent of C or below on an A- F scale. If Harvard can graduate over half a class as Cum Laude, the rest of us can skew grades anywhere we like. Jay Doug Federman wrote: I have a dilemma which I haven't found a good solution for. I work with students who rotate with different preceptors on a monthly basis. A student will have at least 12 evaluations over a year's time. A preceptor usually will evaluate several students over the same year. Unfortunately, the preceptors rarely agree on the grades. One preceptor is biased towards the middle of the 1-9 likert scale and another may be biased towards the upper end. Rarely, does a given preceptor use the 1-9 range completely. I suspect that a 6 from an easy grader is equivalent to a 3 from a tough grader. I have considered using ranks to give a better evaluation for a given student, but I have a serious constraint. At the end of each year, I must submit to another body their evaluation on the original 1-9 scale, which is lost when using ranks. Any suggestions? -- It has often been remarked that an educated man has probably forgotten most of the facts he acquired in school and university. Education is what survives when what has been learned has been forgotten. - B.F. Skinner New Scientist, 31 May 1964, p. 484 = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ = -- Jay Warner Principal Scientist Warner Consulting, Inc. North Green Bay Road Racine, WI 53404-1216 USA Ph: (262) 634-9100 FAX: (262) 681-1133 email: [EMAIL PROTECTED] web: http://www.a2q.com The A2Q Method (tm) -- What do you want to improve today? = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
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hey hey
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Re: chi square validity?
[EMAIL PROTECTED] (Benjamin Kenward) wrote in message news:9vnj9m$s2c$[EMAIL PROTECTED]... Hi folks, Let's say you have a repeatable experiment and each time the result can be classed into a number of discrete categories (in this real case, seven). If a treatment has no effect, it is known what the expected by chance distribution of results between these categories would be. I know that a good test to see if a distribution of results from a particular treatment is different to the expected by chance distribution is to use a chi-squared test. What I want to know is, is it valid to compare just one category? In other words, for both the obtained and expected distributions, summarise them to two categories, one of which is the category you are interested in, and the other containing all the other categories. If the chi-square result of the comparison of these categories is significant, can you say that your treatment produces significantly more results in particularly that category, or can you only think of the whole distribution? Yes, as long as the choice of which category to do it for is not based on the data... no fair just testing the most extreme one. Glen Glen = Instructions for joining and leaving this list and remarks about the problem of INAPPROPRIATE MESSAGES are available at http://jse.stat.ncsu.edu/ =
