Hi Troels, I really cannot follow and judge how the techniques compare. I must be getting old. So to remedy this, I have created the test_suite/shared_data/dispersion/Kjaergaard_et_al_2013/exp_error_analysis/ directory (r25437, http://article.gmane.org/gmane.science.nmr.relax.scm/23187). This contains 3 scripts for comparing R2eff and I0 parameters for the 2 parameter exponential curve-fitting: 1) A simple script to perform Monte Carlo simulation error analysis. This is run with 10,000 simulations to act as the gold standard. 2) A simple script to perform covariance matrix error analysis. 3) A simple script to generate 2D Grace plots to visualise the differences. Now I can see how good the covariance matrix technique is :) Could you please check and see if I have used the relax_disp.r2eff_err_estimate user function correctly? The Grace plots show that the error estimates are currently terrible. Cheers, Edward