[en] An important measure of the opacity of a solid with respect to monoenergetic electrons in a solid is the effective attenuation length (EAL). However, there is much controversy in the literature concerning the definition of this parameter. It has been shown recently that different quantitative applications of electron spectroscopies require EALs resulting from different expressions. In the present report, these expressions for typical applications of X-ray photoelectron spectroscopy and Auger electron spectroscopy are briefly reviewed. The EALs needed for determination of overlayer thicknesses and for measurement of surface composition are compared for the same experimental configuration. These comparisons are made for selected photoelectron lines for which we expect strong electron elastic-scattering effects (Cu2s and Cu2p_3/2 in Cu and Au4s and Au4f_7/2 in Au). Substantial differences between EALs for these lines were found for the two applications. Synchrotron radiation can be used for experimental determination of the EAL values. These values can facilitate evaluation of the reliability of theoretical models used in calculations of EALs