Re: [R] About poisson distribution fitting and testing
The easiest, general function in R that I know for fitting distributions is fitdistr in library(MASS). However, for Poisson, I might also try glm(y~1, family = poisson, ...). Regarding testing for goodness of fit, I just got 44 hits for RSiteSearch(goodness of fit to Poisson). hope this helps. spencer graves p.s. My reply might have been more informative if you had provided example data. For example, for the Poisson, the theoretical variance equals the mean. Clearly, if the sample mean and variance are 6 and 11, that alone could raise questions about the adequacy of the Poisson assumption, depending on the sample size. Linda Lei wrote: Hi All, I have a sequence of positive integers, which is right skewed. The mean value is 6 and variance is 11. I suspect it can be fitted by poisson distribution. But I'm not familiar with the function to fit distribution. Could you please help me with it? Also, once I fit the poisson distribution, how can I check the good-ness of this fitting? Thank you! [[alternative HTML version deleted]] __ 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 __ 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
[R] About poisson distribution fitting and testing
Hi All, I have a sequence of positive integers, which is right skewed. The mean value is 6 and variance is 11. I suspect it can be fitted by poisson distribution. But I'm not familiar with the function to fit distribution. Could you please help me with it? Also, once I fit the poisson distribution, how can I check the good-ness of this fitting? Thank you! [[alternative HTML version deleted]] __ 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
Re: [R] About poisson distribution fitting and testing
Does fitdistr() in package MASS help? Peter Ehlers Linda Lei wrote: Hi All, I have a sequence of positive integers, which is right skewed. The mean value is 6 and variance is 11. I suspect it can be fitted by poisson distribution. But I'm not familiar with the function to fit distribution. Could you please help me with it? Also, once I fit the poisson distribution, how can I check the good-ness of this fitting? Thank you! [[alternative HTML version deleted]] __ 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 __ 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
Re: [R] About poisson distribution fitting and testing
(a) Surely to gumdrops, fitting a ***Poisson*** distribution is as trivial as it gets. There's one parameter to estimate; lambda.hat - x.bar - mean(x). (b) If the mean is 6 and the variance is 11, the data are VERY unlikely to be Poisson. If X is Poisson, var(X) = E(X). No indication of the sample size was given, but let's take a sample size of 30 for the purposes of illustration. In this case the probability of getting a variance of 11 from a Poisson distribution with (population) mean equal to 6 is (approximately, by simulation) 0.004. (c) A chi-squared goodness of fit test will also (almost surely!) tell you that the data are not Poisson. cheers, Rolf Turner [EMAIL PROTECTED] __ 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
