UV flux ratios have strong dependence on the assumed SFH and dust extinction of the galaxies and is only sensitive to the upper end of the high mass IMF. Apart from IMF variation [@Boselli2009; @Meurer2009; @Pflamm-Altenburg2009], stochasticity in SFH [@Boselli2009; @Fumagalli2011; @Guo2016], non-constant SFHs [@Weisz2012], and Lyman leakage [@Relano2012] can provide viable explanations to describe offsets between expected Balmer line to UV flux ratios and observed values.

@Kauffmann2014 used SFRs derived via multiple nebular emission line analysis with the 4000Å break and H$\delta_A$ absorption to probe the recent SFHs of SDSS galaxies with $\mathrm{log_{10}(M_*/M_\odot)<10}$ and infer possibilities for IMF variation. They did not find conclusive evidence for IMF variation, with contradictions in the 4000Å features with @Bruzual2003 stellar templates being attributed to errors in the spectro-photometric calibration. However, using absorption line analysis to probe possible IMF variations in actively star-forming galaxies suffers from strong Balmer line emissions that dominate and fill the absorption features. Furthermore, absorption lines probe older stellar populations, and linking them with current star-formation requires further assumptions about the SFH.

@Smit2016 used SED fitting techniques to probe discrepancies between [H$\alpha$]{} to UV SFRs ratios of $z\sim4-5$ galaxies and local galaxies. They inferred an excess of ionizing photons 