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Well, first the variance of a logistic distribution with scale parameter s is not s^2, but (pi^2 * s^2)/3 (see ?rlogis). So if you want the distribution to have a variance of .25, this implies s should be about sqrt(3*0.25)/pi ~ 0.27567. -- Hong Ooi Senior Research Analyst, IAG Limited 388 George St, Sydney NSW 2000 (02) 9292 1566 -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Doran, Harold Sent: Wednesday, 30 November 2005 7:53 AM To: [email protected] Subject: [R] rlogis() in simulation Dear List: We are generating data such that students are clustered in schools for some item response data for a simulation study. One component of our simulation is to generate measurement error from a logistic distribution with a mean of 0 and standard deviation of 1.7 to match the logistic curve of the Rasch model. We are generating an error term for each of the 40 hypothetical test items a student would respond to. So, we create 40 error terms for each of the N students. This error term is a composite of a "school effect" and a student-specific effect. The school effect is the variance term common to all students in the school whereas the student effect varies over N. We want the variance of the school effect, e_gk, to be .25 and we want the variance of the student-specific effect, e_gik, to be 1.7 - .25 = 1.45 so that the total variance sums to about 1.7. Below is a self contained example of a small portion of the simulation. However, I seem to misunderstand the scale and location parameters in the rlogis function. Can anyone suggest how I might specify the variance of the school effect to be .25 and for the student effect to be 1.45 as I describe above? You can see my erroneous code below. Thanks, Harold ## Note, the subscripts are a little messy as I am changing some notation. But it should be transparent (I hope) N <- 5000 # Number of students J <- 50 # Number of schools N_j <- N/J # Number of students in each school rps <- rep(N_j, J) error <- matrix(numeric(N * 40), ncol = 40) for(i in 1:40){ e_gik <- rlogis(N, 0, sqrt(1.7-.25) ) e_gk <- rep(rlogis(J, 0, sqrt(.25) ), rps) error[,i] <- e_gk + e_gik } Windows XP platform i386-pc-mingw32 arch i386 os mingw32 system i386, mingw32 status major 2 minor 2.0 year 2005 month 10 day 06 svn rev 35749 language R [[alternative HTML version deleted]] ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html _______________________________________________________________________________________ The information transmitted in this message and its attachme...{{dropped}} ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
