Fitting the simulated relaxation data of constant-time CPMG experiments (40 ms) in the 2-site exchange model at two (14.1T, and 18.8T) or three (14.1T, 16.5T, and 18.8T) magnetic fields using approximate analytic solutions15,16 or geoCPMG. The 300 data sets of different CPMG frequencies (νCPMG = 25, 50, 100, 125, 250, 500, 1000 Hz) were simulated. The fit results are plotted against the input values, and those with large standard deviations (S.D. of kex > 100.2) during Monte Carlo sampling are not shown. The coefficient of determination (R2) is calculated for each case. The numbers in the parentheses are the percentages of the data remained after filtering out the results with large standard deviations.