  We study two galaxy samples selected in ultraviolet (UV) and in far-infrared
(FIR) for which the spectral energy distributions (SEDs) from the far UV (FUV)
to the FIR are available. We compare the observed SEDs to modelled SEDs with
several star formation histories (SFHs; decaying star formation rate plus
burst) and attenuation laws (power law + 2175 Angstroem bump). The Bayesian
method allows to estimate statistically the best parameters by comparing each
observed SED to the full set of 82800 models. We reach the conclusion that the
UV dust attenuation cannot be estimated correctly from an SED analysis if the
FIR information is not used. The deduced dispersion is larger than with the FIR
data and the distribution is not symetrically distributed about zero: there is
an over-estimation for UV-selected galaxies and an under-estimation for
FIR-selected galaxies. The output from the analysis process suggests that
UV-selected galaxies have attenuation laws in average similar to the LMC
extinction while FIR-selected galaxy attenuation laws more resemble the MW
extinction law. The dispersion about the average relation in the
Log(Fdust/Ffuv) vs. FUV-NUV diagram (once the main relation with FUV-NUV is
accounted for) is explained by two other parameters: the slope of the
attenuation law and the instantaneous birthrate parameter b_0 for UV-selected
galaxies and the same ones plus the strength of the bump for the FIR-selected
galaxies. We propose a recipe to estimate the UV dust attenuation for
UV-galaxies only (that should be used whenever the FIR information is not
available because the resulting Afuv is poorly defined with an uncertainty of
about 0.32): A_{FUV} = 1.4168 (FUV-NUV)^2 + 0.3298 (NUV-I)^2 + 2.1207 (FUV-NUV)
+ 2.7465 (NUV-I) + 5.8408
