On Mon, Jan 17, 2011 at 9:23 AM, Bert Gunter <gunter.ber...@gene.com> wrote: > Folks: > > I know this may be overreaching, but are we missing what's important? > WHY do the zeros occur? Are they values less then a known or unknown > LOD? -- and/or is there positive mass on zero? In either case, using > logs to calculate a geometric mean may not make sense. Paraphrasing
Isn't this a bit of a general problem with the geometric mean if there are 0s or an odd number of negative numbers it becomes 0 or imaginary (please do correct me if I'm wrong)? sqrt(prod(c(2, 0, 54))) sqrt(prod(c(-2, 2))) > Greg Snow, what is the scientific question? What is the model? > > Cheers, > Bert > > > > On Mon, Jan 17, 2011 at 9:13 AM, Keith Jewell <k.jew...@campden.co.uk> wrote: >> Just in case some of x are negative (the desired median still exists, as >> long as the two middle values are non -ve), how about: >> >> x <- runif(20, -1, 100) >> exp(median(log(pmax(0,x)))) >> >> It'll give -Inf if the two middle values are negative, when I guess we >> should get NaN, but I can't see a 1-line way to handle that! >> >> Keith J >> >> "Peter Ehlers" <ehl...@ucalgary.ca> wrote in message >> news:4d3468ef.5010...@ucalgary.ca... >>> I've been reminded by Prof. Brian Ripley that R's >>> log() function will indeed handle zeros appropriately. >>> >>> Apologies to S Ellison and Hadley Wickham. >>> >>> Peter Ehlers >>> >>> On 2011-01-17 06:55, Peter Ehlers wrote: >>>> On 2011-01-17 02:19, S Ellison wrote: >>>>> Will this do? >>>>> >>>>> x<- runif(20, 1, 100) >>>>> >>>>> exp( median( log( x) ) ) >>>>> >>>>> S Ellison >>>>> >>>>> >>>> That's what Hadley proposed, too. It's fine for >>>> your example, but there is potentially a small >>>> problem with this method: the data must be positive. >>>> Since it's not unusual to see data with some zeros, >>>> the log() would fail. >>>> >>>> Depending on what type of data I was going to use >>>> this modification of the median for, I would consider >>>> modifying the (quite short) median.default function, >>>> with appropriate additional data checks. >>>> >>>> Peter Ehlers >>>> >>>>> >>>>>>>> Skull Crossbones<witch.of.agne...@gmail.com> 15/01/2011 16:26>>> >>>>> Hi All, >>>>> >>>>> I need to calculate the median for even number of data points.However >>>>> instead of calculating >>>>> the arithmetic mean of the two middle values,I need to calculate their >>>>> geometric mean. >>>>> >>>>> Though I can code this in R, possibly in a few lines, but wondering if >>>>> there >>>>> is >>>>> already some built in function. >>>>> >>>>> Can somebody give a hint? >>>>> >>>>> Thanks in advance >>>>> >>> >> >> ______________________________________________ >> R-help@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. >> > > > > -- > Bert Gunter > Genentech Nonclinical Biostatistics > > ______________________________________________ > R-help@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. > -- Joshua Wiley Ph.D. Student, Health Psychology University of California, Los Angeles http://www.joshuawiley.com/ ______________________________________________ R-help@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.
