  Star formation rates (SFR) are compared as determined from mid-infrared 7.7
um PAH luminosity [SFR(PAH)], from 1.4 GHz radio luminosity [SFR(radio)], and
from far ultraviolet luminosity [SFR(UV)] for a sample of 287 starburst
galaxies with z < 0.5 having Spitzer IRS observations. The previously adopted
relation log [SFR(PAH)] = log [vLv(7.7 um)] - 42.57+-0.2, for SFR in solar
masses per year and vLv(7.7 um) the luminosity at the peak of the 7.7 um PAH
feature in ergs per s, is found to agree with SFR(radio). Comparing with
SFR(UV) determined independently from ultraviolet observations of the same
sources with the GALEX mission (not corrected for dust extinction), the median
log [SFR(PAH)/SFR(UV)] = 1.67, indicating that only 2% of the ultraviolet
continuum typically escapes extinction by dust within a starburst. The ratio
SFR(PAH)/SFR(UV) depends on infrared luminosity, with form log
[SFR(PAH)/SFR(UV)]= (0.53+-0.05)log Lir - 4.11+-0.18, for Lir in solar
luminosities. We also find that the ratio SFR(PAH)/SFR(UV) is similar to that
in infrared-selected starbursts for a sample of Markarian starburst galaxies
originally selected using optical classification, which implies that there is
no significant selection effect in SFR(PAH)/SFR(UV) using starburst galaxies
discovered by Spitzer. These results indicate that SFRs determined with
ultraviolet luminosities require dust corrections by a factor of ~ 10 for
typical local starbursts but this factor increases to > 700 for the most
luminous starbursts at z ~ 2.5. With this amount of extinction, the optical
magnitude of a starburst having fv(7.7 um) of 1 mJy should be V ~ 25.6.
