Characteristic X-ray intensity ratios of M and L lines I(M)/I(L) from a series of pure elements were measured in the analytical electron microscope at 75-200kV. Compiling the previous measured ratios together, all the data have been compared with the calculated values from eight cross-section formulas after normalization in order to eliminate the uncertain parameters, leaving only the voltage dependence of the cross-section. These formulas can be divided into four groups for I(L)/I(K) or three groups for I(M)/I(L) ratios. Only one group, containing two formulas suggested by Fabre de la Ripelle and Schreiber and Wims, is in good agreement with all the experimental data.