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 6950
of 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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