Monday, January 02, 2006

RE: st: returning a value from a program

We, meaning the Editors of the Stata Journal, will negotiate with Joe about reissue of the original -cpoisson- and -epoisson- commands from STB-1 under different names. They are, in current terms, immediate commands and would be better labelled as such.

Regardless of when temporary variables were introduced into Stata, using any kind of variables for the calculations tackled by Joe is not essential, and indeed never was.

Nick n.j.cox@durham.ac.uk

Jhilbe@aol.com 21 December 2005 17:38 > I noticed the the discussion regarding exact poisson > probabilities on the > list. I get the digest only, so have had to respond later. > > I did indeed make a mistake with naming two programs by the > same name. The > current -cpoisson is a censored poisson written using version > 9.1 code. It is > found on the SSC site. I had forgotten that I wrote another > back in 1991. The > -cpoisson- I wrote for STB-1 in May 1991 calculated > cumulative poisson > probabilities. However, there is another program that was > published together with > it that calculates exact poisson probabilities, called > -epoisson.ado-. Is > this more of what you want? One simply provides the > population ratio (mu), the > number of observations, and the number of cases. The exact > poisson probability > is thereupon displayed. An example is provided in the help > file, which can be > downloaded from the web. > > I have pasted the file code into this email below. Note that > it uses version > 2.1 code and creates new variables, and at the end erases > all variables in > memory. Like the old -cpoisson- program, it was meant to be > used without > anything else in memory. As I recall, Stata had not yet > developed the temp > variable class. It didn't come out until version 3. And there > may have been problems > using the program if data was already in memory. Instead of > the -drop _all - > code, you can replace it with dropping just those variables > that were > created by the program. Of course, it is preferable to just > rewrite it with > tempvars and in current code. This would eliminate any such > problems. > > Isn't there another function in Stata that calculates exact > and cumulative > poisson probabilities? These were the first two programs > written for the STB > some 16 years ago. I haven't checked, but it seems that Stata > should have a > function for it that can be used in your program. > > Joe Hilbe > > > /* DATE 4\24\91 */ > /* EXACT POISSON PROBABILITY > Joseph Hilbe - STB */ > > program define epoisson > version 2.1 > if "%_3"=="" { > di in re "Type 3 arguments:" > di in re " pop ratio observations cases" > exit 198 > } > drop _all > quietly { > set obs %_3 > if (%_3<1) { > set obs 1 > gen double expect=%_1*%_2 > di in gr "Exact poisson probability => " in ye exp(-expect) > } > if (%_3>0) { > gen double expect=%_1*%_2 > gen double prob=exp(-expect) > gen double cum=1 > gen num=[_n-1]+1 > > while num-1<_N { > replace cum=cum*num > replace num=num+1 > } > replace prob=(expect^%_3)*(exp(-expect))/cum > di in gr "Exact poisson probability => " in ye prob > } > } > drop _all > end

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