  (Abridged) The observational determination of the behaviour of the star
formation rate (SFR) with look-back time or redshift has two main weaknesses:
1- the large uncertainty of the dust/extinction corrections, and 2- that
systematic errors may be introduced by the fact that the SFR is estimated using
different methods at different redshifts. To assess the possible systematic
differences among the different SFR estimators and the role of dust, we have
compared SFR estimates using H$\alpha$, SFR(H$\alpha$), [OII]$\lambda$3727\AA,
SFR(OII), UV, SFR(UV) and FIR, SFR(FIR) luminosities of a sample comprising the
31 nearby star forming galaxies having high quality photometric data in the UV,
optical and FIR.
  We review the different "standard" methods for the estimation of the SFR and
find that while the standard method provides good agreement between
SFR(H$\alpha$) and SFR(FIR), both SFR(OII) and SFR(UV) are systematically
higher than SFR(FIR), irrespective of the extinction law.
  We show that the excess in the SFR(OII) and SFR(UV) is mainly due to an
overestimate of the extinction resulting from the effect of underlying stellar
Balmer absorptions in the measured emission line fluxes. Taking this effect
into consideration in the determination of the extinction brings the SFR(OII)
and SFR(UV) in line with the SFR(FIR) and simultaneously reduces the internal
scatter of the SFR estimations. Based on these results we have derived
"unbiased" SFR expressions for the SFR(UV), SFR(OII) and SFR(H$\alpha$). We
have used these estimators to recompute the SFR history of the Universe using
the results of published surveys.
