R help - Oct 2013 - Small p from binomial probability function.

Hi,

I got given some code that uses the R function pbionom:

p <- mut * t
sumprobs <- pbinom( N, B, p ) * 1000

Which gives the output of a probability as a percentage like 5, 50, 95.

What the code currently does is find me the values of t I need, by using the
above two code lines in a loop, each iteration it increaces t by one and runs
the two lines. When sumprobs equals 5, it records the value t, then again when
sumprobs is equal to 50, and again when sumprobs is equal to 95 - giving me
three t values. This is not an efficient way of doing this if t is large. Is it
possible to rearrange pbinom so it gives me the small p (made of mut*t) as the
result of plugging in the sumprobs instead, and is there an R function that
already does this?

Since pbinom is the binomial probability equation I suppose the question is - in
more mathematical terminology - can I change this code so that instead of
calculating the Probability of N successes given the number of trials and the
probability of a single success, can I instead calculate the probability of a
single success using the probability of N successes and number of trials, and
the number of successes? Can R do this for me. So instead I plug in 5, 50, and
95, and then get the small p out?

Thanks,
Ben W.





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Sounds like you want a 95% binomial confidence interval:

	binom.test(N, P)

will compute this for you, and you can get the bounds directly with

	binom.test(N, P)$conf.int

Actually, binom.test computes a two-sided confidence interval, which corresponds
roughly to 2.5 and 97.5 percentages in your approach. It doesn't give you
the 50% point either, but I don't think that's a meaningful quantity
with a two-sided test.

Hope this helps,
Stefan


On 9 Oct 2013, at 15:53, Benjamin Ward (ENV) <B.Ward at uea.ac.uk> wrote:
> I got given some code that uses the R function pbionom:
> 
> p <- mut * t
> sumprobs <- pbinom( N, B, p ) * 1000
> 
> Which gives the output of a probability as a percentage like 5, 50, 95.
> 
> What the code currently does is find me the values of t I need, by using
the above two code lines in a loop, each iteration it increaces t by one and
runs the two lines. When sumprobs equals 5, it records the value t, then again
when sumprobs is equal to 50, and again when sumprobs is equal to 95 - giving me
three t values. This is not an efficient way of doing this if t is large. Is it
possible to rearrange pbinom so it gives me the small p (made of mut*t) as the
result of plugging in the sumprobs instead, and is there an R function that
already does this?
> 
> Since pbinom is the binomial probability equation I suppose the question is
- in more mathematical terminology - can I change this code so that instead of
calculating the Probability of N successes given the number of trials and the
probability of a single success, can I instead calculate the probability of a
single success using the probability of N successes and number of trials, and
the number of successes? Can R do this for me. So instead I plug in 5, 50, and
95, and then get the small p out?