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From |
Philip Ryan <philip.ryan@adelaide.edu.au> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
RE: st: probability mass function for a binomial distribution |

Date |
Mon, 30 Jun 2008 11:21:25 +0930 |

<sacrifice>

I note [using Stata 10, latest] that -help Binomial- returns help on -binomial()- not on -binomialtail()-

I believe in other circumstances Stata returns an "out of date" message for functions and commands whose names have changed (e.g. -help index- )

Phil

At 03:07 AM 30/06/2008, you wrote:

Readers following along should note the renaming of various functions in this territory from Stata 9 to Stata 10. Stata 10 users can see -help whatsnew9to10-. That is, Carlo, as he signalled, is using Stata 9. Stata 9's -Binomial()- is Stata 10's -binomialtail()-. -Binomial()- continues to work. Thus I think this appearance of being undocumented is in fact a side-effect of name change. Nick n.j.cox@durham.ac.uk Steven Samuels I agree with Nick's advice to construct the functions on first principles whenever possible. Interestingly, Carlo has discovered the apparently undocumented "survival' function Binomial(k,n,p) equal to P(X>=k), whereas the documented function binomial(k,n,p) is P(X<=k). Here is a version of Carlo's program with a correction for k= 20. -Steve /*---------------------------begin example-------------------------*/ drop _all clear set obs 21 g id=_n-1 g n=20 g k=_n-1 g p=.2 g double Binomial=Binomial( n, k, p) g double PMF_Binomial=Binomial[_n]- Binomial[_n+1] in 1/20 replace PMF_Binomial=Binomial[_n] in 21 //Correction g double PMF2 = 0.2^k * 0.8^(20 - k) * comb(20, k) list id n k p Binomial binomial PMF* /*--------------------------end example----------------------------*/ On Jun 29, 2008, at 11:26 AM, Nick Cox wrote: > What is available as a defined function shows up a trade-off > problem. It > wouldn't be difficult to define a thousand functions, but then some > people might complain about the complexity of the list and the > difficulty of finding a solution. > > Otherwise put, I guess the answer to Carlo's question is that Stata > users -- unlike spreadsheet users, it seems --- are paid the > compliment > of knowing enough statistics to work this out from first principles: > > gen double bmp = p^k * (1 - p)^(20 - k) * comb(20, k) > > Note in passing two other details: > > I prefer to use -double-s here. > > Putting constants into variables isn't necessary: > > gen double bmp = 0.2^k * 0.8^(20 - k) * comb(20, k) > > In cases like this the advantage of a canned function over a one-line > solution using another canned function would be pretty small. > > Nick > n.j.cox@durham.ac.uk > > Carlo Lazzaro > > I have probably found out the answer to my last Friday thread. > I do hope this may be useful for someone else on the list. > However, a little concern remains about the lack among Stata 9.2/SE > commands > of an in-built for calculating the (probably too trivial) probability > mass > function for a binomial distribution, which is instead available > within > the > most widespread spreadsheets. > One more time, my grateful thought go out to Steve and Nick (in > order of > appearance in my incoming e-mail folder) for pointing me out the > importance > of a step-by-step research in dealing with Stata troubles. > > ---------------------------begin example------------------------- > set obs 21 > g id=_n-1 > g n=20 > g k=_n-1 > g p=.2 > g Binomial=Binomial( n, k, p) > g PMF_Binomial=1- Binomial if id==20 > replace PMF_Binomial=Binomial[_n]- Binomial[_n+1] in 1/20 > --------------------------end example---------------------------- > * * 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/

Philip Ryan Professor, Discipline of Public Health Director, Data Management & Analysis Centre Associate Dean (IT) Faculty of Health Sciences postal address: Discipline of Public Health Mail Drop 511 University of Adelaide 5005 South Australia location: Level 6, Room 6-18 Bice Building Royal Adelaide Hospital North Terrace Adelaide tel +61 8 8303 3570 fax +61 8 8223 4075 http://www.public-health.adelaide.edu.au/ CRICOS Provider Number 00123M ----------------------------------------------------------- This email message is intended only for the addressee(s) and contains information that may be confidential and/or copyright. If you are not the intended recipient please notify the sender by reply email and immediately delete this email. Use, disclosure or reproduction of this email by anyone other than the intended recipient(s) is strictly prohibited. No representation is made that this email or any attachments are free of viruses. Virus scanning is recommended and is the responsibility of the recipient. * * 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/

**References**:**R: st: probability mass function for a binomial distribution***From:*"Carlo Lazzaro" <carlo.lazzaro@tin.it>

**RE: st: probability mass function for a binomial distribution***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**Re: st: probability mass function for a binomial distribution***From:*Steven Samuels <sjhsamuels@earthlink.net>

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