Spectral analyses are the basis of estimating spectroscopic properties of rare-earth ions doped materials and Judd-Ofelt (J-O) theory is considered the foundation of spectral analyses. The process calculating three intensity parameters according to J-O model and some important radiative parameters are detailed; the reasonable applications of related formulas are discussed; the originations of errors in calculations are summarized, and a different approach is recommended to obtain more reliable radiative parameters. Experimental oscillator strengths are suggested to be calculated by transmission spectra. The light reflection, scattering and the inherent absorption of matrix should be subtracted in absorption coefficient and cross-section calculations. The line shape factor must be taken into account for determining the average wavenumber and wavelength. The line shape of measured emission spectrum is proposed to be considered for calculating emission cross-sections at different wavelength. However, the errors are usually relatively high, even leading to unreliable results, because many approximations and assumptions were involved in J-O model. It is put forward that if measured radiative lifetimes and branching ratios at low temperature are used to calculate parameters, obtained results will be more reliable and meaningful.