Thank you - I wasn't aware of this function. One can even use lchoose which allows really huge arguments (more than 2^1000)!
--- "Lucke, Joseph F" <[EMAIL PROTECTED]> wrote: > C is an R function for setting contrasts in a > factor. Hence the funky > error message. > ?C > > Use choose() for your C(N,k) > ?choose > > choose(200,2) > 19900 > > choose(200,100) > 9.054851e+58 > > N=200; k=100; m=50; p=.6; q=.95 > choose(N,k)*p^k*(1-p)^(N-k)*choose(k,m)*q^m*(1-q)^(k-m) > 6.554505e-41 > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf > Of Moshe Olshansky > Sent: Wednesday, August 15, 2007 2:06 AM > To: sigalit mangut-leiba; r-help > Subject: Re: [R] binomial simulation > > No wonder that you are getting overflow, since > gamma(N+1) = n! and 200! > (200/e)^200 > 10^370. > There exists another way to compute C(N,k). Let me > know if you need this > and I will explain to you how this can be done. > But do you really need to compute the individual > probabilities? May be > you need something else and there is no need to > compute the individual > probabilities? > > Regards, > > Moshe. > > --- sigalit mangut-leiba <[EMAIL PROTECTED]> wrote: > > > Thank you, > > I'm trying to run the joint probabilty: > > > > C(N,k)*p^k*(1-p)^(N-k)*C(k,m)*q^m*(1-q)^(k-m) > > > > and get the error: Error in C(N, k) : object not > interpretable as a > > factor > > > > so I tried the long way: > > > > gamma(N+1)/(gamma(k+1)*(gamma(N-k))) > > > > and the same with k, and got the error: > > > > 1: value out of range in 'gammafn' in: gamma(N + > 1) > > 2: value out of range in 'gammafn' in: gamma(N - > k) .... > > > > Do you know why it's not working? > > > > Thanks again, > > > > Sigalit. > > > > > > > > On 8/14/07, Moshe Olshansky > <[EMAIL PROTECTED]> > > wrote: > > > > > > As I understand this, > > > P(T+ | D-)=1-P(T+ | D+)=0.05 > > > is the probability not to detect desease for a > > person > > > at ICU who has the desease. Correct? > > > > > > What I asked was whether it is possible to > > mistakenly > > > detect the desease for a person who does not > have > > it? > > > > > > Assuming that this is impossible the formula is > > below: > > > > > > If there are N patients, each has a probability > p > > to > > > have the desease (p=0.6 in your case) and q is > the probability to > > > detect the desease for a person who > > has > > > it (q = 0.95 for ICU and q = 0.8 for a regular > > unit), > > > then > > > > > > P(k have the desease AND m are detected) = P(k > have the desease)*P(m > > > > are detected / k have > > the > > > desease) = > > > C(N,k)*p^k*(1-p)^(N-k)*C(k,m)*q^m*(1-q)^(k-m) > > > where C(a,b) is the Binomial coefficient "a > above > > b" - > > > the number of ways to choose b items out of a > > (when > > > the order does not matter). You of course must > > assume > > > that N >= k >= m >= 0 (otherwise the probability > > is > > > 0). > > > > > > To generate such pairs (k infected and m > detected) > > you > > > can do the following: > > > > > > k <- rbinom(N,1,p) > > > m <- rbinom(k,1,q) > > > > > > Regards, > > > > > > Moshe. > > > > > > --- sigalit mangut-leiba <[EMAIL PROTECTED]> > > wrote: > > > > > > > Hi, > > > > The probability of false detection is: P(T+ | > D-)=1-P(T+ | > > > > D+)=0.05. > > > > and I want to find the joint probability > > > > P(T+,D+)=P(T+|D+)*P(D+) > > > > Thank you for your reply, > > > > Sigalit. > > > > > > > > > > > > On 8/13/07, Moshe Olshansky > > <[EMAIL PROTECTED]> > > > > wrote: > > > > > > > > > > Hi Sigalit, > > > > > > > > > > Do you want to find the probability P(T+ = t > > AND > > > > D+ = > > > > > d) for all the combinations of t and d (for > > ICU > > > > and > > > > > Reg.)? > > > > > Is the probability of false detection (when > > there > > > > is > > > > > no disease) always 0? > > > > > > > > > > Regards, > > > > > > > > > > Moshe. > > > > > > > > > > --- sigalit mangut-leiba <[EMAIL PROTECTED]> > > > > wrote: > > > > > > > > > > > hello, > > > > > > I asked about this simulation a few days > > ago, > > > > but > > > > > > still i can't get what i > > > > > > need. > > > > > > I have 2 units: icu and regular. from icu > I > > want > > > > to > > > > > > take 200 observations > > > > > > from binomial distribution, when > probability > > for > > > > > > disease is: p=0.6. > > > > > > from regular I want to take 300 > observation > > with > > > > the > > > > > > same probability: p=0.6 > > > > > > . > > > > > > the distribution to detect disease when > > disease > > > > > > occurred- *for someone from > > > > > > icu* - is: p(T+ | D+)=0.95. > > > > > > the distribution to detect disease when > > disease > > > > > > occurred- *for someone from > > > > > > reg.unit* - is: p(T+ | D+)=0.8. > > > > > > I want to compute the joint distribution > for > > > > each > === message truncated === ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
