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st: RE: rnd discussion |

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Sat, 29 Sep 2007 00:05:53 EDT |

Nick et al: You provided a nice discussion of rndbin. A few comments might help to understand why and how these random number generators (RNGs) came to be as they are. First, going back to 1993, Stata had only a couple of random number generators. Larry Hamilton had written several others (T, F, and the like) as I recall in his "Statistics with Stata" for version 3. Since I had a need to use other random number generators, I decided to write a complement of them for my own work - and then decided that others might find them useful as well. I asked Walter Linde-Zwirble, a physicist turned health outcomes analyst friend of mine to participate. He wrote the beta binomial RNG and helped test the others. When these random number generators were written in 1993, Stata was in version 3 as I recall. The programming language of Stata was quite different from now. The idea was to use the generators when no other data was in memory. I believe that version 3 required this. Anyhow, the programs were re-written in 1995, but the change involved the manner in which temporary variables were identified. The logic of the programs was retained. It would have taken lots of work to redo them entirely. It is also important to realize that I fully expected that Stata would re-write them and include them in the next release. Stata was the only major Stat package without a compliment of random number generators. I was mistaken.Over 10 years later and they still do not have them as part of the package. I created two types of RNGs. One, generators that simply created a single variable with the distributional properties defined by the user on the command line. Assuming no data in memory, the number of observations and the mean, and scale if appropriate, were specified by the user after the command name. i rarely use these. The second type have an x attached to the end of the RNG, eg rndpoix. This command allows one to create artificial data sets. I have continually used these. After specifying the number of observations, one creates one of more normal random numbers, assigns parameter values to them, plus a value for the constant, and runs the RNG. A data set emerges with the same parameters as defined. How to do this is detailed using -help rnd-. I definitely would have paid more attention to enhancing speed, and perhaps re-writing the algorithm (which use the covering method) if I would have known that Stata was not going to write ones for the official package. As it was, they served a good purpose. At times a Stata user suggested a change, which we made and substituted for the older one on my directory. Most were put on the SSC site on 1997. If you are interested in creating artificial data sets for GLM families (Gaussian, binomial, Poisson, negative binomial, gamma, and inverse Gaussian), Roberto Gutierrez (Stata Corp) wrote a suite of programs for this purpose. The logic of the commands is somewhat close to my rnd programs for the same purpose, but I actually like them better. For the binomial RNG, type -net search genbinomial-. These were the RNGs used for the chapter on Overdispersion in Hardin & Hilbe, Generalized Linear Models and Extensions, 2nd edition (2007, Stata Press), and in my recently released book, Negative Binomial Regression (2007, Cambridge Univ. Press). Using my rnd programs or Roberto's will give the same results. I like Roberto's because you can define the generated variable rather than have it predetermined by the program. This point was mentioned by Nick. However, it was not originally a problem since I assumed no other data was in memory. Stata should seriously consider implementing RNGs in the next release. Mine work fine given the caveats mentioned by Nick. Roberto's are fine as well. But they are limited to GLM families for the purpose of constructing artificial data sets in the spirit of my rndx commands. The other RNGs could well be written by the very capable Stata programmers. Constructing them so that users can create artificial data sets would seem to me the ideal way to go. Roberto has already done much of the work. Joseph Hilbe ************************************** See what's new at http://www.aol.com * * For searches and help try: * http://www.stata.com/support/faqs/res/findit.html * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

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