The Senate Inquiry into Supermarket Prices has escalated into a debate about the merits of return on equity (ROE) as a measure of profitability. We certainly prefer ROE and other returns measures over percentage profit margins. However, in the case of Woolworths, the ROE of 27% (pre sig items) is influenced by historical cost accounting, buybacks and demergers. Care needs to be taken in looking at a single year.