On 05-Aug-2014 10:27:54 Frederico Mestre wrote: > Hello all: > > Is it possible to generate quasi-random positive numbers, given a standard > deviation and mean? I need all positive values to have the same probability > of selection (uniform distribution). Something like: > > runif(10, min = 0, max = 100) > > This way I'm generating random positive numbers from a uniform > distribution. However, using runif I can't previously select SD and mean > (as in rnorm). > > Alternatively, I'm able to generate a list of quasi-random numbers given a > SD and a mean. > > b <- (sqrt(SD^2*12)+(MEAN*2))/2 > a <- (MEAN*2) - b > x1 <- runif(N,a,b) > > However, negative values might be included, since "a" can assume a negative > value. > > Any help? > > Thanks, > Frederico
There is an inevitable constraint on MEAN and SD for a uniform ditribution of positive numbers. Say the parent distribution is uniform on (a,b) with a >= 0 and b > a. Then MEAN = (a+b)/2, SD^2 = ((b-a)^2)/12, so 12*SD^2 = b^2 - 2*a*b + a^2 4*MEAN^2 = b^2 + 2*a*b + a^2 4*MEAN^2 - 12*SD^2 = 4*a*b MEAN^2 - 3*SD^2 = a*b Hence for a >= 0 and b > a you must have MEAN^2 >= 3*SD^2. Once you have MEAN and SD satisfying this constraint, you should be able to solve the equations for a and b. Hoping this helps, Ted. ------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@wlandres.net> Date: 05-Aug-2014 Time: 11:46:52 This message was sent by XFMail ______________________________________________ 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.