[R] Sigmoidal Fitting

[Cressoni, Massimo (NIH/NHLBI) [F]]

I want to thank professor Tura for his help but the function in some cases fails to converge and I am not able to
understand the reason :

> GAS
[1] 0.8108649 1.0386906 1.1638837 2.4144286
> PRESSURE
[1]  0  5 15 45
>  model.1 <- nls(GAS_PER_G ~ SSlogis(PRESSURE, ASym, xmid, scal))
 
 model.1 <- nls(GAS_PER_G ~ SSlogis(PRESSURE, ASym, xmid, scal))
Error in nls(y ~ 1/(1 + exp((xmid - x)/scal)), data = xy, start = list(xmid = aux[1],  : 
	step factor 0.000488281 reduced below 'minFactor' of 0.000976562
> 

Massimo Cressoni