Call:
sfa(formula = log(N) ~ log(D), data = mydata)
Maximum likelihood estimates
(Intercept) log(D) sigmaSq gamma
6.554390 0.351602 0.003051 0.879910
> summary(fit)
Error Components Frontier (see Battese & Coelli 1992)
Inefficiency decreases the endogenous variable (as in a production function)
The dependent variable is logged
Iterative ML estimation terminated after 14 iterations:
cannot find a parameter vector that results in a log-likelihood value
larger than the log-likelihood value obtained in the previous step
Multiplied the initial values 2 time(s) by 0.999 before the search procedure could start
You could try to use different starting values or try to reduce the step size specified in argument 'searchStep'