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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: RE: generate lognormal RV less than 20000 observations. |

Date |
Sun, 11 Apr 2010 17:53:10 +0100 |

Sun Samn made two points. First, "Sorry, I just updated my rndlgn; the old one does require 20000 or more." There is some misunderstanding by Sun here. For a long time, the -rnd- package by Joseph Hilbe and W. Linde-Zwirble was the most developed way to get random deviates in Stata. -rnd- includes -rndlgn-. Typing . findit rndlgn reveals the facts. Search of official help files, FAQs, Examples, SJs, and STBs STB-41 sg44.1 . . . . . . . . . . . . Correction to random number generators (help rnd if installed) . . . . . . . . J. Hilbe and W. Linde-Zwirble 1/98 p.23; STB Reprints Vol 7, p.166 faster version plus minor changes STB-28 sg44 . . . . . . . . . . . . . . . . . . . . Random number generators (help rnd if installed) . . . . . . . . J. Hilbe and W. Linde-Zwirble 11/95 pp.20--21; STB Reprints Vol 5, pp.118--121 programs to implement random number generators that allows user to generate synthetic data from a variety of distributions Web resources from Stata and other users (contacting http://www.stata.com) sg44_1 from http://www.stata.com/stb/stb41 STB-41 sg44_1. Correction to random number generators. / STB insert by / Joseph Hilbe, Arizona State University; / Walter Linde-Zwirble, Health Outcomes Technologies. / Support: atjmh@asuvm.inre.asu.edu / After installation, see help rnd. sg44 from http://www.stata.com/stb/stb28 STB-28 sg44. Random number generators. / STB insert by / Joseph Hilbe, / Walter Linde-Zwirble. / Support: atjmh@asuvm.inre.asu.edu / After installation, see help rnd. rnd from http://www.stata.com/users/jhilbe rnd. Random data generators. / Program by Joseph Hilbe, Arizona State Univ. <jhilbe@aol.com> / Statalist distribution, 29 January 1999. / These programs generate random numbers for a variety of important / distributions. This is an update of my STB-28 insert. / See help rnd. rnd from http://fmwww.bc.edu/RePEc/bocode/r 'RND': modules for random number generation / The random number generators in this package are updated versions / of those published in STB-28 and STB-44. They allow for / noncentrally distributed random numbers, distributed F, t, or / Chi-squared. The other RNGs include some which are -rndlgn- was published in STB-28 and updated in STB-41. There is another copy on the StataCorp website. The most up-to-date public version is on SSC. As I said, there has never been any such requirement of -rndlgn- to have 20000 or more observations. I don't know where such a bizarre idea comes from. The help gives various examples of using -set obs- before you call the -rnd- commands. Independently of that, the -rnd- programs will increase the dataset size if the number of observations specified is greater than the number of observations in memory. But the treatment of dataset size has not changed at all in the public lifetime of -rndlgn-, so there is no difference between older and newer versions of -rndlgn- in this respect. The most notable change between STB-28 and SSC versions is a correction of a typo in the comment: the third argument is the standard deviation, not the variance. . type http://www.stata.com/stb/stb28/sg44/rndlgn.ado *!version 1.0.0 1993 Joseph Hilbe, Walter Linde-Zwirble (sg44: STB-28) * Lognormal distribution random number generator [via normal distribution] * Example: rndlgn 1000 0 .5 [set obs 1000; 0 = mean; .5 = variance] program define rndlgn version 3.1 cap drop xlgn qui { local cases `1' set obs `cases' mac shift local mn `1' mac shift local var `1' mac shift tempvar ran1 noi di in gr "( Generating " _c gen `ran1' = exp(`mn'+`var' * invnorm(uniform())) gen xlgn = `ran1' noi di in gr "." _c noi di in gr " )" noi di in bl "Variable " in ye "xlgn " in bl "created." } end . ssc type rndlgn.ado *!version 1.1 1999 Joseph Hilbe * version 1.0.0 1993 Joseph Hilbe (sg44: STB-28) * Lognormal distribution random number generator [via normal distribution] * Example: rndlgn 1000 0 .5 [set obs 1000; 0 = mean; .5 = std. deviation] program define rndlgn version 4.0 set type double cap drop xlgn qui { local cases `1' set obs `cases' mac shift local mn `1' mac shift local var `1' mac shift tempvar ran1 noi di in gr "( Generating " _c gen `ran1' = exp(`mn'+`var' * invnorm(uniform())) gen xlgn = `ran1' noi di in gr "." _c noi di in gr " )" noi di in bl "Variable " in ye "xlgn " in bl "created." lab var xlgn "Lognormal random variable" set type float } end Second, " For my case, I prefer to use rndlgn instead of exp(normal) because I wanna [meaning:want to] specify heteroskedasticity." There is no real difference here. In Stata 10.1 up, you can call -rnormal()- with specified mean and standard deviation; either or both could be given via variable names, or indeed more complicated expressions. This is quite as flexible as, indeed slightly more flexible than, using -rndlgn-, as the latter reliably supports only single numbers or variable names as arguments. Nick n.j.cox@durham.ac.uk sun samn Sorry, I just updated my rndlgn; the old one does require 20000 or more. For my case, I prefer to use rndlgn instead of exp(normal) because I wanna specify heteroskedasticity. n.j.cox@durham.ac.uk > This sparked a thread with Martin Weiss. I have four comments by way of summarizing and going beyond that exchange. > > 1. Please do specify where user-written software you refer to comes from. > (Most postings from Sun Samn ignore advice in the FAQ in at least one way.) > > 2. It is quite untrue that -rndlgn- requires 20,000 observations. > > 3. In this case, the user-specified software is not, and never has been, needed. A one-line call with reference to -exp(rnormal())- gets you random draws from a lognormal quite directly. (Before -rnormal()- was introduced, other functions could be used, and indeed were used internally within -rndlgn-. I imagine that for their own reasons Joe Hilbe and friend wanted something uniform in syntax with the other commands that they wrote a while back.) > > 4. -rndlgn- is a command, not a function. -exp()- and -rnormal()- are functions. sun samn > I know the function ' rndlgn' can generate lognormal RVs, but it requires the numbers of observation to be at 20000. Now, I want a list of only 500. What should I do then? * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: record the running time of program***From:*sun samn <shwshang@hotmail.com>

**Re: st: record the running time of program***From:*Maarten buis <maartenbuis@yahoo.co.uk>

**RE: st: record the running time of program***From:*sun samn <shwshang@hotmail.com>

**st: generate lognormal RV less than 20000 observations.***From:*sun samn <shwshang@hotmail.com>

**st: RE: generate lognormal RV less than 20000 observations.***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**RE: st: RE: generate lognormal RV less than 20000 observations.***From:*sun samn <shwshang@hotmail.com>

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