In mle estimation, we can constrain the permissible space for the parameters under estimation like this: mle logl = check ? p*ln(ax) - lngamma(p) - ln(x) - ax : NA series ax = alpha*x scalar check = (alpha>0) && (p>0) params alpha p end mle Do we use the same syntax when using nls, non-linear least-squares? The general difference in syntax that I can see, is that in mle we define directly the function that is to be maximized, while in nls we define the regression function... so maybe the syntax for the constraint should be different? -- Alecos Papadopoulos PhD Candidate Athens University of Economics and Business, Greece School of Economic Sciences Department of Economics https://alecospapadopoulos.wordpress.com/