Shinichi Nakagawa
2009-Jul-14 23:35 UTC
[R] Simulation functions for underdispered Poisson and binomial distributions
Dear R users I would like to simulate underdispersed Poisson and binomial distributions somehow. I know you can do this for overdispersed counterparts - using rnbinom() for Poisson and rbetabinom() for binomial. Could anyone share functions to do this? Or please share some tips for modifying existing functions to achieve this. Thank you very much for your help and time Shinichi ____________________ Shinichi Nakagawa, PhD (Lecturer of Behavioural Ecology) Department of Zoology University of Otago 340 Great King Street P. O. Box 56 Dunedin, New Zealand Tel: +64-3-479-5046 Fax: +64-3-479-7584 http://www.otago.ac.nz/zoology/staff/academic/nakagawa.html
Charles C. Berry
2009-Jul-15 16:01 UTC
[R] Simulation functions for underdispered Poisson and binomial distributions
On Wed, 15 Jul 2009, Shinichi Nakagawa wrote:> Dear R users > > I would like to simulate underdispersed Poisson and binomial distributions > somehow. > > I know you can do this for overdispersed counterparts - using rnbinom() for > Poisson and rbetabinom() for binomial. > > Could anyone share functions to do this? Or please share some tips for > modifying existing functions to achieve this.Shinichi, You really need a model for the underdispersion. Using that model, you would calculate the probabiltities fo the binomial or Poisson counts. But you have to come up with something appropriate for your situation. For example, probs <- prop.table( dbinom( 0:10, 10, .5)^3 ) or probs <- prop.table( dbinom( 0:10, 10, .5) + ifelse( 0:10 == 5, 1, 0) ) will each produce probabilities for counts that are less dispersed than probs <- dbinom( 0:10, 10, 0.5 ) but neither may suitably model the counts for the situation in which you are interested. --- Once you have that model in hand sample( 0:10, N, pr=probs, repl=TRUE ) will 'simulate' N such counts. HTH, Chuck> > Thank you very much for your help and time > > Shinichi > ____________________ > Shinichi Nakagawa, PhD > (Lecturer of Behavioural Ecology) > Department of Zoology > University of Otago > 340 Great King Street > P. O. Box 56 > Dunedin, New Zealand > Tel: +64-3-479-5046 > Fax: +64-3-479-7584 > http://www.otago.ac.nz/zoology/staff/academic/nakagawa.html > > ______________________________________________ > R-help at r-project.org 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. >Charles C. Berry (858) 534-2098 Dept of Family/Preventive Medicine E mailto:cberry at tajo.ucsd.edu UC San Diego http://famprevmed.ucsd.edu/faculty/cberry/ La Jolla, San Diego 92093-0901