Dear All,
 
I have a questions I would like to ask about and wonder if you have any thoughts to make it work in R.
 
1. I work in the field of medicine where physiologic variables are often simulated, and they can not have negative values. Most often the assumption is made to simulate this parameters with a normal distribution but in the "log-domain" to avoid from negative values to be generated. Since the expected mean  and SD is usually known from the normal domain, using the methods described in the wikipedia article "Arithmetric moments" I generate μand σ and simulate with rlnorm(). At times though the following issue comes up: I have the mean and SD for the parameters available from the normal domain, and the covariance matrix from the normal domain. Then I would like to simulate the values, but to avoid from negative values being generated I have to fall back on rlnorm in {compositions}. My issue is though that my covariance matrix is representing the covariance of the parameters in the normal domain, as opposed to in the lognormal domain. Any thoughts on how
 to work around this?
 
apreciate the help,
 
Andras
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