[ii]{}\], \[S [ii]{}\],\[O [i]{}\] doublets and narrow H$\alpha$) having been subtracted, 400 mock double-peaked lines can be created by the bootstrap method. When the bootstrap method is applied, we should avoid the selection of some adjacent data points to create mock lines, otherwise, the following estimated uncertainty should be unreasonably small, due to few variation of line profile. Then line widths of the 400 mock lines are measured by Equation (3). After that, standard deviation of the 400 measured line widths of the mock lines is accepted as the uncertainty of the line width of the observed double-peaked broad H$\alpha$, and listed in Table 1. Certainly, we should note that Peterson et al. (2004) accepted $\sigma_{err}/\sigma=81{\rm km/s}/3185{\rm km/s}\sim2.5\%$ as the mean uncertainty of the line width of broad H$\beta$ of 3C390.3 within observed wavelength range from 5006Åto 5246Å. It is consistent with our value $<\sigma_{err}/\sigma>\sim2.2\%$, which indicates our estimated uncertainties for the line width of all the 67 double-peaked broad H$\alpha$ are reasonable and acceptable.

Besides the line width and the corresponding uncertainty, it is not difficult to accomplish spectral flux calibration for 3C390.3. However, because the spectra of 3C390.3 are observed by different instruments in different observatories under different 