[jimnotgym]

I really hate the terminology of the first sentence

> Divide a company's income by its revenue

How about dividing a companies operating profit by its revenue? Income is a vague term and is just as often equated with revenue... which makes the opening sentence a bit weird.

Going further, most people talking about different sectors having different margins are talking about the gross profit margin. In a retailer gross profit could be the sales minus the cost of the things that got sold and probably the cost of the people in the stores. In a service business it is normally the sales minus the cost of people doing the work that was sold. At a hosting company it could be the sales- minus the electricity, Internet, engineers.

The important distinction is that gp does not normally include 'head office costs', accountants and other parasites, so it is easier to compare the different segments from the amount they are going to contribute towards your fixed costs.