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Re: st: gamma


From   Maarten Buis <maartenlbuis@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: gamma
Date   Thu, 11 Oct 2012 15:00:29 +0200

There is a difference between the gamma function
(http://en.wikipedia.org/wiki/Gamma_function) and the gamma
distribution (http://en.wikipedia.org/wiki/Gamma_distribution).
-gamma- used the gamma distribution in a survival analysis context,
what you seem to want is the gamma function. Nick already told you how
to get that, so there is no need to go back and forth between Stata
and R.

-- Maarten

On Thu, Oct 11, 2012 at 2:45 PM, Jason Rosenberg <RSNJAS002@myuct.ac.za> wrote:
> When I type in: help gamma, I get:
>
> Out-of-date command
>     As of Stata 8.0, the cox, weibull[het], lnormal[het], gamma[het], llogist,
>     llogistic[het], ereg[het], and gompertz[het] commands are out of date.
>     The replacement for cox is stcox.
>     The replacement for weibull[het], lnormal[het], gamma[het], llogist,
>     llogistic[het], ereg[het], and gompertz[het] is streg.
> I read through those but to no avail.
>
> In any case I used R to do the gamma part.
> If possible could you tell how to run this for a continuous variable in place of the 6, I did it for discreet variables, as in:
> egen number = seq()
> then:
> gen prob=(v/gamma(alpha/v))*(beta^(2*alpha)/(number+11.2)^(alpha+1))*exp(-(beta^2/(number+11.2))^v)
> but I need a curve instead of a simple scatter plot. If you could direct me to the relevant help files that would suffice..
>
>
> ________________________________________
> From: owner-statalist@hsphsun2.harvard.edu [owner-statalist@hsphsun2.harvard.edu] on behalf of Nick Cox [njcoxstata@gmail.com]
> Sent: Thursday, October 11, 2012 12:31 PM
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: gamma
>
> There is no function -gamma()-, largely I suspect because its values
> become very big.
>
> There is a function -lngamma()-, so you would need -exp(lngamma())-.
>
> Almost all of this can be established by looking at the help for -functions-.
>
> Nick
>
> On Thu, Oct 11, 2012 at 1:25 PM, Jason Rosenberg <RSNJAS002@myuct.ac.za> wrote:
>> hi
>> I an attempting to put this equation into stata:
>>
>> gen prob=(v/gamma(alpha/v))*(beta^(2*alpha)/(6+11.2)^(alpha+1))*exp(-(beta^2/(6+11.2))^v)
>> where v, alpha, and beta are predefined my the user
>> and I get :
>> unknown function gamma()
>>
>> Is there another syntax I should know for gamma?
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-- 
---------------------------------
Maarten L. Buis
WZB
Reichpietschufer 50
10785 Berlin
Germany

http://www.maartenbuis.nl
---------------------------------

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