Breaking up the ROE analysis is one way to do it. However, the better way is to look at a set of ratios and put them together for a consolidated picture. Obviously, none of the profitability ratios can be looked at in isolation. They will have to be looked at with reference to the other ratios too. But there are some basic thumb rules. If the net profit margins are higher due to other income or extraordinary items, then it is not viewed as very positive from a valuation point of view. On the hand, NPM that is purely driven by improvement in OPM is much more meaningful from the valuation perspective.
OPMs are a function of yield enhancement and cost control. For example, when cement companies get the pricing power back in their hands, the yields get enhanced and therefore the OPMs move up. Alternatively, look at the case of paint companies. For paints, crude oil constitutes a very important production input. Since crude oil prices have been moving down since November 2014, paint companies have seen their OPMs expand due to better cost control. Either way, markets are more interested in the know whether this OPM improvement is sustainable or not!