Sales-Cost of goods sold = Gross Margin
Gross Margin - Operation cost =Earning B4 Interest and Tax (Profit Margin)
Earning b4 Interest and Tax - interest and tax expenses = Net Income
see the Definition from http://www.investorwords.com/cgi-bin/getword.cgi?2944
too complicated, there are various margin. profit margin is one type. but margin is for sure not equal to margin ratio. Make sense?