er provides a simple way to estimate the errors in the parameters. The algorithm is very simple, and the answer it gives is intuitively satisfying, but not statistically rigorous. The algorithm varies one parameter at a time and finds two values of the parameter where chisqr is increased by the given factor. An increase of 10 - 20% in chisqr should give a reasonable estimate of the errors in the parameter, so I suggest a factor of 1.1 or 1.2. For factors larger than 2.0, the algorithm may not converge, and you answers would not make sense as error estimates on the parameters.