[R] Binomial

[Alexander Engelhardt]

Am 12.05.2011 13:19, schrieb Sarah Sanchez:
> Dear R helpers,
>
> I am raising one query regarding this "Binomial" thread with the sole intention of learning something more as I understand R forum is an ocean of knowledge.
>
> I was going through all the responses, but wondered that original query was about generating Binomial random numbers while what the R code produced so far generates the Bernoulli Random no.s i.e. 0 and 1.
>
> True Binomial distribution is nothing but no of Bernoulli trials. As I said I am a moron and don't understand much about Statistics. Just couldn't stop from asking my stupid question.

Oh, yes.
You can generate one B(20,0.7)-distributed random varible by summing up 
the like this:

 > pie <- 0.7
 > x <- runif(20)
 > x
  [1] 0.83108099 0.72843379 0.08862017 0.78477878 0.69230873 0.11229410
  [7] 0.64483435 0.87748373 0.17448824 0.43549622 0.30374272 0.76274317
[13] 0.34832376 0.20876835 0.85280612 0.93810355 0.65720548 0.05557451
[19] 0.88041390 0.68938009
 > x <- runif(20) < pie
 > x
  [1] FALSE FALSE  TRUE FALSE  TRUE  TRUE  TRUE  TRUE FALSE  TRUE FALSE 
  TRUE
[13] FALSE FALSE  TRUE  TRUE FALSE  TRUE FALSE FALSE
 > sum(x)
[1] 10

You could shorten this to

 > sum(runif(20)<0.7)
[1] 12

Which would be the same as

 > rbinom(1,20,0.5)
[1] 6

or even

 > qbinom(runif(1),20,0.5)
[1] 12


Just play around a little, and learn from the help files:

 > ?rbinom

Have fun!

>
> --- On Thu, 5/12/11, David Winsemius<dwinsemius at comcast.net>  wrote:
>
> From: David Winsemius<dwinsemius at comcast.net>
> Subject: Re: [R] Binomial
> To: "Alexander Engelhardt"<alex at chaotic-neutral.de>
> Cc: r-help at r-project.org, "blutack"<x-jess-h-x at hotmail.co.uk>
> Date: Thursday, May 12, 2011, 11:08 AM
>

>
> I hope Allan knows this and is just being humorous here,  but for the less experienced in the audience ... Choosing a different threshold variable name might be less error prone. `pi` is one of few built-in constants in R and there may be code that depends on that fact.
>
>> pi
> [1] 3.141593

He didn't, or better, he forgot.
Also, that Allan isn't related to me (I think) :)

  - Alex