From: Gunther Höning <gunther.hoening_at_ukmainz.de>

Date: Tue, 01 Jul 2008 14:40:44 +0200

R-help_at_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. Received on Tue 01 Jul 2008 - 12:54:20 GMT

Date: Tue, 01 Jul 2008 14:40:44 +0200

Dear list,

I have some values like

Time 2 4 8 24 48 72

UTR 82543 169105 207615 96633 31988 7005 UTRs 82687 172934 205541 101842 31898 6950of a twice repeated meassurement.

I know that the underlying function is of :

f(x) = SO/(sqrt(2*pi)*sigma *x) * exp(-(S0*ln x - mu)^2/(2*sigma^2)).

How can I determine the value of S0, sigma and mu.

I tried nls:

lognormal <- function(x,S0,sigma,mu) {S0/(sqrt(2*pi)*sigma *x) *
exp(-(S0*log (x) - mu)^2/(2*sigma^2))}

x <- rep(c(2,4,6,24,48,72),2)

y <-

c(82543,169105,207615,96633,31988,7005,82687,172934,205541,101842,31898,6950
)

dat <- data.frame(x,y)

regfit <- nls(y ~ lognormal(x,S0,sigma,mu) ,data =dat)

Output:

Error in nlsModel(formula, mf, start, wts) :

singular gradient matrix at initial parameter estimates
Warning message:

No starting values specified for some parameters.
Intializing 'S0', 'sigma', 'mu' to '1.'.
Consider specifying 'start' or using a selfStart model in: nls(y ~
lognormal(x, S0, sigma, mu), data = dat)

Can anybody help on this topic please ?

Gunther

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