Sales$-$COGS$=$GM$-$SG&A$=$NOI $($Sales less cost of goods sold$=$ gross margin less selling general & admin expenses $=$ net operating income$)$ This is generally accepted accounting principles. Now a new concept is introduced in Chapter $6-$ Contribution Margin $($CM$).$ What is the contribution margin? Is a CM different than a GM$.$ If so$,$ how? If a GM is generally accepted accounting, what's the purpose of a CM$?$ What is a better way to look at an income statement, using CM approach or GM approach?$$