Re: st: peculiar issue generating truncated normals

[Patrick Roland <patrick.rolande@gmail.com>]

You could pick the r1's for all trials, and then the r2's for all
trials. What I meant by saying that procedure is sequential is that
you need to draw r2 after drawing r1.

I've replicated this on Stata 12.

On Fri, Jan 13, 2012 at 10:45 AM, Nick Cox <n.j.cox@durham.ac.uk> wrote:
> I haven't tried it, but it seems that you should be able to able to get r1 all at once, then r2, etc. I could be wrong...
>
> In terms of your main questions, please confirm that you are using Stata 12 or updated Stata 11.2. A small problem with the -rnormal()- functions was uncovered during Stata 11.
>
> Nick
> n.j.cox@durham.ac.uk
>
> Patrick Roland
>
> For the accept-reject sampler I am drawing a vector of results all at
> once. The other procedure is inherently sequential.
>
> This result surprises me because the random number generators in Stata
> are meant to pass all kinds of tests for randomness. Yet it seems that
> this is a test of randomness they don't pass...
>
> On Fri, Jan 13, 2012 at 12:38 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>> This does not particularly surprise me given what you are doing, but
>> that is a visceral reaction. You seem to be going for the tails of the
>> distribution.
>>
>> A different issue is that getting results one by one does not seem
>> essential: you can get a vector of results all at once and -- when
>> appropriate -- reject unwanted values all at once. Mata supports
>> elementwise operations with vectors and matrices.
>>
>> On Fri, Jan 13, 2012 at 4:03 AM, Patrick Roland
>> <patrick.rolande@gmail.com> wrote:
>>> I've been trying two ways of generating truncated multivariate normals
>>> in mata, which give different results. I'd be interested if anyone
>>> could shed light on why this might be. In the code I generate
>>> bivariate normal draws with variance c*c', where c = (1,0\1,2) ,
>>> truncated above at (-2,-2).
>>>
>>> In the first case, I use simple accept reject sampling. In the second,
>>> I sequentially draw truncated normals (this is explained here, for
>>> example: http://www.hss.caltech.edu/~mshum/gradio/ghk_desc.pdf).
>>>
>>> The means are consistently different, across many different seeds. The
>>> first variable has a mean of around -2.41 with accept reject and -2.37
>>> with sequential sampling. I'm running Stata SE 11.2. This seems
>>> peculiar to me - any ideas?
>>>
>>> mata
>>> u = (-2,-2)
>>> trials = 100000
>>> c = (1,0\1,2)
>>> GHK = J(trials,2,0)
>>> for(i=1;i<=trials;i++){
>>> r1 = invnormal(runiform(1,1)*normal(u[1]/c[1,1]))
>>> r2 = invnormal(runiform(1,1)*normal((u[2]-c[2,1]*r1)/c[2,2]))
>>> GHK[i,.] = (c*(r1\r2))'
>>> }
>>>
>>> i=0
>>> AR = J(trials,2,0)
>>> while(i<trials){
>>>        t = c*rnormal(2,1,0,1)
>>>        if(t'<u){
>>>                i=i+1
>>>                AR[i,.] = t'
>>>        }
>>> }
>>>
>>> mean(GHK)
>>> variance(GHK)
>>> mean(AR)
>>> variance(AR)
>>> end
>>> *
>
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