In our opinion, the model proposed by Mock et al. (2007), with the choice of assurance provider as dependent variable and selected criteria of assurance quality as predictors, is not appropriate. We argue instead that the analysis should reversely focus on the differential effects of assurance provider’s choice on varying level of assurance quality. Furthermore, we refine their model by classifying the quality of a sustainability assurance statement using the evaluative framework proposed by O’Dwyer and Owen (2005). Three categories of assurance quality are identified, namely “reporting format”, “assurance procedures” and “recommendations and opinion”. A description of the categories is provided in the variable measurement section.