Abstract It has been observed that the ratio of Hα to FUV luminosity (L H α /L FUV ) is lower in low surface brightness galaxies. This behaviour has been attributed to systematic variations of the upper mass end and/or the slope of the Initial Mass Function (IMF) Meurer et al . (2009) and Lee et al . (2009)). However these hypotheses do not explain the observed scatter in luminosity ratio (L H α /L FUV ). We present a model for the total L H α and L FUV luminosity arising from a randomly populated IMF following the Salpeter power law and the clustering law of Oey & Clarke (2007).