proc nlin - input model function

[pablorodriguez1]

I am trying to find a functino that find the best-fit function according to some data points that I have. I am doing that with proc nlin, and have succeeded at computing it with a logarithmic model function. Currently I am trying to do it with a different model function* but I haven't managed to make it work.

I think I may be inputting the function with the wrong expression.

* The model function is the Weibull distribution whose expression is f(t) = 1 - e^(-(t/a)^b);

PROC NLIN DATA=aux_&s
	MAXITER=500
	CONVERGE=1E-05
	SINGULAR=1E-08
	MAXSUBIT=30
	OUTEST=coeficientes_segmento&s 
	NOITPRINT
	;
	model segmento11 = (1 - (EXP(-(t/a)**b))); *Weibull distribution function;
	;
	PARMS
	a=0.4
	b=0.5
;
RUN;

[Ksharp]

http://blogs.sas.com/content/iml/2015/06/08/fit-circle.html