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Re: Which is faster? ziggurat or Monty Python (or maybe something else?)
The Box-Muller algorithm rejects roughly 22.5% of the generated points. I'm not aware of any bound on the number of consecutive rejections, other than a statistical one, hence my statement. I would welcome correction if this is not the case. Regards Ian Radford Neal [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Box-Muller does not work for real time requirements. This isn't true, of course. A real time application is one where one must guarantee that an operation takes no more than some specified maximum time. The Box-Muller method for generating normal random variates does not involve any operations that could take arbitrary amounts of time, and so is suitable for real-time applications. This assumes that the time needed for Box-Muller is small enough, which will surely often be true. If the time allowed is very small, then of course one might need to use some other method. Rejection sampling methods would not be suitable for real-time applications, since there is no bound on how many points may be rejected before one is accepted, and hence no bound on the time required to generate a random normal variate. Radford Neal = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Correlations-statistics
Hi, I have calculated correlation coefficients between sets of parameters (A) and (B) and beween (A) and (C). Now I would like to determine the correlation between (A) and (B combined with C). How can I combine the two parameters (B) and (C), what kind of statistical method has to be applied? Thanks for your tips, Holger Boehm = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Efficient convergence method
Hi, I am writing a computer simulation, and I really would appreciate some advice about statistics side of things! Each simulation run has fixed settings, but there is some randomness involved (e.g. start position). As a result, each simulation scenario needs to be run until the universal mean (say time taken for objective to be met) varience is reduced. The simulation has just one output that needs measurement - time taken, and there is no transient state. The question is, what accuracy is acceptable, and how can I guartee that the varience is small enough to be accurate, while being efficient on computing power. Any methods, techniques etc. gladly welcome, as I am new to stats!! Thanks. = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Efficient convergence method
Hi, I am writing a computer simulation, and I really would appreciate some advice about statistics side of things! Each simulation run has fixed settings, but there is some randomness involved (e.g. start position). As a result, each simulation scenario needs to be run until the universal mean (say time taken for objective to be met) varience is reduced. The simulation has just one output that needs measurement - time taken, and there is no transient state. The question is, what accuracy is acceptable, and how can I guartee that the varience is small enough to be accurate, while being efficient on computing power. Any methods, techniques etc. gladly welcome, as I am new to stats!! Thanks. = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Numerical recipes in statistics ???
I do not think this will satisfy on its own but it seems a good starting point. SPSS has up on its web pages the statistical algorithms for many of its procedures: http://www.spss.com/tech/stat/algorithms.htm As these are not the computer algorithmns (at least I hope not) I think you will need to chase the references at the end of the algorithmns you are interested it. Jean The Truth [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Glen Barnett [EMAIL PROTECTED] wrote in message news:[EMAIL PROTECTED]... The Truth wrote: Are there any Numerical Recipes like textbook on statistics and probability ? Just wondering.. What do you mean, a book with algorithms for statistics and probability or a handbook/cookbook list of techniques with some basic explanation? Glen I suppose I should have been more clear with my question. What I essentially require is a textbook which presents algorithms like Monte Carlo, Principal Component Analysis, Clustering methods, MANOVA/MANACOVA methods etc. and provides source code (in C , C++ or Fortran) or pseudocode together with short explanations of the algorithms. There are books on statistical computing that cover some algorithms (usually with pseudocode rather than actual source code), but to cover all of statistics is not possible. The particular subset you suggest above are not all covered in any one book I have seen. You should be able to find books that cover some Monte Carlo techniques and regression and maybe bootstrapping and a few other basic techniques - stuff that goes somewhat beyond what's in NR, but not nearly as far as you seem to be after. You can find code for many of these things (and much more besides) in journals like JRSS C (Applied Statistics), and a few others (e.g. ACM Transactions on Mathematical Software). A lot of these algorithms are on the Internet. Glen = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ = -- Jean M. Russell M.A. M.Sc. [EMAIL PROTECTED] Corporate Information Computing Services, University of Sheffield 285 Glossop Road Sheffield S10 2HB United Kingdom Phone: 0114-222-3098 Fax : 0114-222-3040 = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Which is faster? ziggurat or Monty Python (or maybe something else?)
Ian Buckner [EMAIL PROTECTED] wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
The Box-Muller algorithm rejects roughly 22.5% of the
generated points. I'm not aware of any bound on the number
of consecutive rejections, other than a statistical one, hence
my statement. I would welcome correction if this is not the case.
Regards
Ian
Radford Neal [EMAIL PROTECTED] wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
Box-Muller does not work for real time requirements.
This isn't true, of course. A real time application is one where
one must guarantee that an operation takes no more than some
specified
maximum time. The Box-Muller method for generating normal random
variates does not involve any operations that could take arbitrary
amounts of time, and so is suitable for real-time applications.
This assumes that the time needed for Box-Muller is small enough,
which will surely often be true. If the time allowed is very small,
then of course one might need to use some other method.
Rejection sampling methods would not be suitable for real-time
applications, since there is no bound on how many points may be
rejected before one is accepted, and hence no bound on the time
required to generate a random normal variate.
Radford Neal
=
What is usually called the Box-Muller method is
the transformation of normal variates to polar coordinates
that we all owe to Laplace for showing us how to evaluate
\int_0^\infty e^{-x^2}. To get a pair of independent
standard normal variates x,y, use two [0,1) uniform variates
U1,U2 and put
x = sqrt(-2*ln(U1))*cos(2pi*U2)
y = sqrt(-2*ln(U1))*sin(2pi*U2).
This is a fixed-time procedure.
The random-time procedure, often mistakenly
called Box-Muller, was developed in 1956
by Marsaglia:
Generate uniform (-1,1) variates V1,V2 until
S = V1^2 + V2^2 1
then return
x = V1*sqrt(-2ln(S)/S)
y = V2*sqrt(-2ln(S)/S).
George Marsaglia
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Re: Question on random number generator
Herman Rubin wrote: ExpVar = -ln(UnifVar); It is not a good method in the tails, and is much too slow. If I recall correctly, transcendental operations on a Pentium require only a couple hundred clock cycles and can usually be optimized to take place during other calculations; so a few million simulations per second ought to be possible on the average domestic machine. -Robert Dawson = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Correlations-statistics
well, one simple way would be to add B and C ... then correlate with A if these are radically different scales, convert to z scores first At 02:05 AM 2/20/02 -0800, Holger Boehm wrote: Hi, I have calculated correlation coefficients between sets of parameters (A) and (B) and beween (A) and (C). Now I would like to determine the correlation between (A) and (B combined with C). How can I combine the two parameters (B) and (C), what kind of statistical method has to be applied? Thanks for your tips, Holger Boehm = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ = Dennis Roberts, 208 Cedar Bldg., University Park PA 16802 Emailto: [EMAIL PROTECTED] WWW: http://roberts.ed.psu.edu/users/droberts/drober~1.htm AC 8148632401 = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Normalization procedures
Hello everybody, Has anybody heard of the Bell-Doksum test? If so could you please give me a reference or a short descriptiion of it. Some one mentioned it to me and it's driving me crazy not to be able to find any info either on the net or in my references. Best Niko Tiliopoulos = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Evaluation of skating
Next question: How much does Rasch analysis depend upon the evaluators being ignorant that the method will be used? In other words, can (A) one Rasch-aware judge (B) a minority of Rasch-aware judges (C) a majority of Rasch-aware judges (but not the whole panel) still (a) bring about a desired result (b) without giving scores stranger than are now given? (c) without needing a specially-programmed Palm Pilot or other smoking gun to hack the math? ` -Robert Dawson = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
How to test whether f(X,Y)=f(X)f(Y) is true??
Hi! I have some experimental data collected and can be grouped into 2 variables, X and Y. One is the dependent variable (Y) and the other is an independent variable (X). What test shall I made to check whether there can be expressed as independent or not?? Thanks.. Linda = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Normalization procedures
?? It is well known. Haven't you checked the Combined Index to Statistics, or even looked in The Encyclopedia of Statistical Sciences? Niko Tiliopoulos wrote: Hello everybody, Has anybody heard of the Bell-Doksum test? If so could you please give me a reference or a short descriptiion of it. Some one mentioned it to me and it's driving me crazy not to be able to find any info either on the net or in my references. Best Niko Tiliopoulos -- Bob Wheeler --- (Reply to: [EMAIL PROTECTED]) ECHIP, Inc. = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
SPC control limits
Hi all, hope this is the right place. i was just wondering what you should do when you establish some control limits but some of the data points you've just used are outside of the limits you just established? should you write them off as bad, leave them or go back and see/fix the problem? any help appreciated, hope the above makes some sense. -steve- = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
What is an experiment ?
Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? Thanks for any comments. Voltolini _ Prof. J. C. VOLTOLINI Grupo de Estudos em Ecologia de Mamiferos - ECOMAM Universidade de Taubate (UNITAU) Departamento de Biologia Taubate, SP, Brasil. CEP 12030-010 Tel: 0XX12-2254165 (lab.), 2254277 (secret. depto.) FAX: 12 - 2322947 E-Mail: [EMAIL PROTECTED] _ = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
At 03:59 PM 2/20/02 -0300, Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? the main issue is CONTROL OVER ... that the experimenter exerts over an independent variable if you want to compare diversity of fish ACROSS rivers ... and you find a difference, what does this necessarily have to do with contamination? i see three variables here ... diversity of fish ... rivers ... level of contamination (ie, where the gradients are different) what are you trying to show impacts on what? Dennis Roberts, 208 Cedar Bldg., University Park PA 16802 Emailto: [EMAIL PROTECTED] WWW: http://roberts.ed.psu.edu/users/droberts/drober~1.htm AC 8148632401 = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: How to test whether f(X,Y)=f(X)f(Y) is true??
You can start with checking if they are correlated. It's simpler to do. If you find that they are correlated then you have the answer to your question. If you find that they are uncorrelated and you have a reason to believe that they may be not independent anyway then you can look for more advanced tests. On 20 Feb 2002, Linda wrote: Hi! I have some experimental data collected and can be grouped into 2 variables, X and Y. One is the dependent variable (Y) and the other is an independent variable (X). What test shall I made to check whether there can be expressed as independent or not?? Thanks.. Linda = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Efficient convergence method
the real question is, 'how much accuracy (precision, variance) is
suitable?'
If you were to repeat the simulation run (i.e., a test) a total of n
times, then you could say that the true mean elapsed time was x-bar +/-
(certain amount), with say 95% confidence.
That is, if you were to then repeat the whole process, n times again, 95%
of the time the x-bar would fall within the +/- (certain amount) you had
calculated. The average of your mean elapsed time is probably Normal, so
this equation can be used. If you want to predict the one next elapsed
time from the next simulation run, then you have to believe that your
individual times are Normally distributed, or do some deeper analysis.
If that's confusing, I'm sorry, but it comes from what you asked.
You can do the simulation run n times, and _estimate_ a value for mean
elapsed time that could be confirmed only by say 100*n runs. Does this
sound like what you want?
The eq. for the 'certain amount' is given by
certain amount = s*z/sqrt(n)
where s = stdev of your n run times, z = 1.96 for 95% confidence, and n =
number of simulation runs.
Pick a confidence interval ('certain amount') that you like, then solve
for n to decide how many runs you will need to make. Statistics cannot
tell you what confidence interval is suitable to your problem - that is a
technical issue. It can tell you now many n's you need to reach that
confidence interval.
Is this what you were looking for?
Cheers,
Jay
PS:Yes, I know 'accuracy' and 'precision' refer to different things.
But you used the first of these words in a way which I infer meant the
latter, so I opened the first sentence in that manner.
Gooseman wrote:
Hi,
I am writing a computer simulation, and I really would appreciate some
advice about statistics side of things!
Each simulation run has fixed settings, but there is some randomness
involved (e.g. start position). As a result, each simulation scenario
needs to be run until the universal mean (say time taken for objective
to be met) varience is reduced.
The simulation has just one output that needs measurement - time
taken, and there is no transient state.
The question is, what accuracy is acceptable, and how can I guartee
that the varience is small enough to be accurate, while being
efficient on computing power. Any methods, techniques etc. gladly
welcome, as I am new to stats!!
Thanks.
=
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--
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?
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Re: Correlations-statistics
[EMAIL PROTECTED] (Holger Boehm) wrote in message news:[EMAIL PROTECTED]... Hi, I have calculated correlation coefficients between sets of parameters (A) and (B) and beween (A) and (C). Now I would like to determine the correlation between (A) and (B combined with C). How can I combine the two parameters (B) and (C), what kind of statistical method has to be applied? Thanks for your tips, Holger Boehm If b and c are not correlated at all then your coefficients should be the same.. = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: SPC control limits
On 20 Feb 2002 08:50:19 -0800, [EMAIL PROTECTED] (MrTequila) wrote: Hi all, hope this is the right place. i was just wondering what you should do when you establish some control limits but some of the data points you've just used are outside of the limits you just established? should you write them off as bad, leave them or go back and see/fix the problem? - looks like an early chance to test them. If you go back and there is *not* any problem, then here's a hint that your limits may not be useful after all. I suppose, if you *know* you did badly, you could write yourself a failing grade -- based on guilt-feelings rather than on data -- and change the limits without ever testing whether there is a real problem. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: How to test whether f(X,Y)=f(X)f(Y) is true??
Vadim and Oxana Marmer [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... You can start with checking if they are correlated. It's simpler to do. If you find that they are correlated then you have the answer to your question. If you find that they are uncorrelated and you have a reason to believe that they may be not independent anyway then you can look for more advanced tests. Can you give some examples of more advanced tests that can be used to test the depedency of data when there these data are uncorrelated?? Thanks.. CCC On 20 Feb 2002, Linda wrote: Hi! I have some experimental data collected and can be grouped into 2 variables, X and Y. One is the dependent variable (Y) and the other is an independent variable (X). What test shall I made to check whether there can be expressed as independent or not?? Thanks.. Linda = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: can multicollinearity force a correlation?
My tentative conclusion is that your 2% effect really is a small one; it should be difficult to discern among likely artifacts; and therefore, it is hardly worth mentioning I agree to me it makes sense as well: fasting insulin should have more to do with error and genetics than food and exercise, I'm not giving up though. I've tried transforming Insulin as I noted odd error behavior on my residuals but it only improved R^2 marginally. Also I don't know if the fact that my population is so large is making a difference. I note that most published studies usually study percentiles of serum levels. This makes more sense I think as maybe 10,000 people will have normal serum levels whereas 400 might have abnormal, and so would this have an effect on r^2. I think I am breaking the assumption of regression that you can't repeat the same points over and over. I will try to Consolidate people into groups and then re-run the data. I'm not sure if this will make a difference, but this is how i see it done in the literature. Statistics is interesting, it is hard to find information on the problems you come across and they can only be tackled by running more queries from different angles.. an exception : i asked a while ago whether standardized beta coefficients are valid and the answer was shown to be no, curiously i came across a journal article on this very topic, if anyone was following the article is A heuristic method for estimating the relative weight of predictor variables in multiple regression (Multivr behav res. 35 1 1-19, 2000) This article is very intereting to read... much to comment.. = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
What is an experiment ?
I think also that an experiment is the human attempt to make sense out of the chaotic world: the method is you assume chaos, H0 and then disprove it.. so you don't need controls because the experiment can be run to prove maybe that the equation for velocity is valid.. (validation experiment).. (ie you can disprove the null hypothesis, chaos by showing that somehthing always occurs).. I had an argument with a collegue: I said that B may not cause disease because there was no proof. They said that B did cause disease because there was no proof against it. Basically I think I'm write in that you always start with assuming chaos and no relation between disease and exposure and then you do your experiment.. no controls are needed, i think because sometimes you are just describing, as Jay pointed out.. like with testing the velocity equation, or discovering another relationship (equation etc.) = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: How to test whether f(X,Y)=f(X)f(Y) is true??
On Wed, 20 Feb 2002 19:30:19 -, Chia C Chong [EMAIL PROTECTED] wrote: Vadim and Oxana Marmer [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... You can start with checking if they are correlated. It's simpler to do. If you find that they are correlated then you have the answer to your question. If you find that they are uncorrelated and you have a reason to believe that they may be not independent anyway then you can look for more advanced tests. Can you give some examples of more advanced tests that can be used to test the depedency of data when there these data are uncorrelated?? You can check for an obvious non-linear (say quadratic) fit. WHAT is your 'reason to believe that they may be not independent'? Anything that makes any pattern, at all, is 'dependent.' So there is an infinite variety of tests conceivable. So the *useful* test is the one that avoids 'Bonferroni correction, because it is the one you perform because you have some reason for it. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? It's not an experiment, but not for the reason you are thinking. The reason it's not an experiment is because there has been no assignment of exposure or intervention to the units being investigated; rather, levels of the exposure (pollution) occur naturally among the groups being investigated (fish populations of each river). Hence, your study is not experimental, but rather, observational. As to the subject of control groups: there is no rule that says a control group has to have a zero level of the exposure. You can use, for instance, the river with the lowest pollution level as your control group (reference group might be a better name) and compare the effects of higher levels of pollution to that group. If you can quantify pollution levels, you could also model the effect of pollution level as a continuous variable in such a way that the lowest level of pollution in the study would implicitly be the reference level. The problem that you have in any observational study, however, is with drawing causal inferences from the results. Any observed association between pollution level and species diversity could at least logically be caused by any characteristic of the rivers that differed among them and that was associated with their pollution levels. If there are such extraneous differences, they must be controlled in some manner in order to make valid inferences about the effect of pollution on species diversity. -Jay = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: What is an experiment ?
Voltolini wrote: Hi, I was reading a definition of experiment in science to be used in a lecture and the use of treatments and controls are an important feature of an experiment but my doubt is... is it possible to plan an experiment without a control and call this as an experiment ? For example, in a polluted river basin there is a gradient of contamination and someone are interested in to compare the fish diversity in ten rivers of this basin. Then, the pollution level are the treatment (with ten levels) but if there is not a clean river in the basin, I cannot use a control ! Is this an experiment anyway ? It's not an experiment, but not for the reason you are thinking. The reason it's not an experiment is because there has been no assignment of exposure or intervention to the units being investigated; rather, levels of the exposure (pollution) occur naturally among the groups being investigated (fish populations of each river). Hence, your study is not experimental, but rather, observational. As to the subject of control groups: there is no rule that says a control group has to have a zero level of the exposure. You can use, for instance, the river with the lowest pollution level as your control group (reference group might be a better name) and compare the effects of higher levels of pollution to that group. If you can quantify pollution levels, you could also model the effect of pollution level as a continuous variable in such a way that the lowest level of pollution in the study would implicitly be the reference level. The problem that you have in any observational study, however, is with drawing causal inferences from the results. Any observed association between pollution level and species diversity could at least logically be caused by any characteristic of the rivers that differed among them and that was associated with their pollution levels. If there are such extraneous differences, they must be controlled in some manner in order to make valid inferences about the effect of pollution on species diversity. -Jay = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Numerical recipes in statistics ???
[EMAIL PROTECTED] (The Truth) wrote: I suppose I should have been more clear with my question. What I essentially require is a textbook which presents algorithms like Monte Carlo, Principal Component Analysis, Clustering methods, MANOVA/MANACOVA methods etc. and provides source code (in C , C++ or Fortran) or pseudocode together with short explanations of the algorithms. You might try looking for books which are specifically related to S+. Why is that? Well, the S language is very high-level, so that algorithms remain comprehensible even in their implementation. Also, the R implementation of S (www.r-project.org) is an open source package which is very powerful. One can say the same about Matlab -- in this case the alternative implementation is Octave (www.octave.org). There is a book by Venables Ripley titled something like Statistical Programming in S+. I don't have it, but I've read other works by Ripley, so I believe it will be a good book. I believe you'll find many other S+-centric books. StatLib (lib.stat.cmu.edu) is a very large collection of algorithms and data sets -- you can learn a lot by browsing and downloading materials from there. Hope this helps, Robert Dodier = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: How to test whether f(X,Y)=f(X)f(Y) is true??
Rich Ulrich [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... On Wed, 20 Feb 2002 19:30:19 -, Chia C Chong [EMAIL PROTECTED] wrote: Vadim and Oxana Marmer [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... You can start with checking if they are correlated. It's simpler to do. If you find that they are correlated then you have the answer to your question. If you find that they are uncorrelated and you have a reason to believe that they may be not independent anyway then you can look for more advanced tests. Can you give some examples of more advanced tests that can be used to test the depedency of data when there these data are uncorrelated?? You can check for an obvious non-linear (say quadratic) fit. WHAT is your 'reason to believe that they may be not independent'? Anything that makes any pattern, at all, is 'dependent.' I have an example of data of 2 RVs. When I tested the correlation between them, by simply find the correlation coefficient, it shows that the correlation coefficient is so small and therefore, I could say that these two RVs are uncorrelated,or better still, not linearly correlated. However, when I plotted the scatter plot of them, it is clearly shown that one of the varriable does dependent on the other variable in some kind of pattern, is just that there are not lineraly dependent, hence the almost zero correlation coeffiicent. So, I am just wonder whether any kind of tests that I could use to test dependency between 2 varaibles... CCC So there is an infinite variety of tests conceivable. So the *useful* test is the one that avoids 'Bonferroni correction, because it is the one you perform because you have some reason for it. -- Rich Ulrich, [EMAIL PROTECTED] http://www.pitt.edu/~wpilib/index.html = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: How to test whether f(X,Y)=f(X)f(Y) is true??
Linda [EMAIL PROTECTED] wrote in message [EMAIL PROTECTED]">news:[EMAIL PROTECTED]... Hi! I have some experimental data collected and can be grouped into 2 variables, X and Y. One is the dependent variable (Y) and the other is an independent variable (X). What test shall I made to check whether there can be expressed as independent or not?? There are so many ways variables can fail to be independent that a truly general test usually won't have good power against specific alternatives. Essentially you'd need to estimate f(Y|X) somehow and compare it to f(Y) (also estimated somehow). I have no advice on the best way to tackle the test, since it depends on how you do the estimation (and you need to keep in mind that since the two distributions are estimated from the same data, they are not independent). If XY are categorical, there are a number of general tests of independence, of which the usual Pearson chi-squared test of independence is the best known. It's much better if you can specify the kind of alternatives you care about most, and the more specific the better. For example, one thing that would help to nail it down a little would be to say you only care about relationship in the mean - i.e. you need to detect if E(Y|X) = E(Y). This is still very general, but it's better. If you're only interested in monotonic relationships, it's easier still. But you need to clarify what you require. Glen = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Re: Chi-square chart in Excel
Ronny Richardson wrote: Can anyone tell me how to produce a chart of the chi-square distribution in Excel? (I know how to find chi-square values but not how to turn those into a chart of the chi-square curve.) Ronny Richardson = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ = I assume you want the pdf, not the cdf. Set up a column of x's (e.g. 0,0.2, 0.4, ...), and beside it set up a column of pdf values (type in the pdf for the chisq you're after as a function of x): For m d.f.: 1/[Gamma(m/2)*2^(m/2)]*x^(m/2-1)*exp(-x/2) (in excel you'll need exp(gammaln()) because it doesn't have a Gamma function.) Note that you can set up m in a cell, so you can play around with the d.f. and see what it does to the curve. So now you have 2 columns you can plot. Click on the chart icon, choose the XY(scatter) plot option, pick either the joined with lines or joined with a curve pictures (without the points marked - either of the rightmost plots there). Choose any other options you need, and there you go. Glen = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
Convert VHS tapes to DVD
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Re: Normalization procedures
Niko Tiliopoulos wrote: Hello everybody, Has anybody heard of the Bell-Doksum test? IIRC it's like a Wilcoxon 2-sample test, except that the ranks are transformed to normal scores. If that's the right test, it has ARE 1 vs the t-test (it has good power for small deviations), but as you move to larger deviations, its power curve flattens out short of 1. Checking the internet: ... 9. Bell, C. B.; Doksum, K. A. Some new distribution-free statistics. Ann. Math. Statist 36 1965 203--214. ... 12. Bell, C. B.; Doksum, K. A. Optimal one-sample distribution-free tests and their two-sample extensions. Ann. Math. Statist. 37 1966 120--132. ... it would just about have to be one of these two papers. Glen = Instructions for joining and leaving this list, remarks about the problem of INAPPROPRIATE MESSAGES, and archives are available at http://jse.stat.ncsu.edu/ =
