South Africa, Nov. 13 -- That last line worries me most.

A comfortable number

CPM is a comfortable number. It's simple, easy to compare, and it falls nicely when buyers negotiate hard or move budget into open programmatic exchanges. A plan that delivers twice the impressions for the same money looks like good stewardship.

But a CPM only tells you what you paid for a thousand ad impressions. It tells you nothing about who, or what, was on the other end.

It's like a market stall selling oranges at half the price of the greengrocer across the road. A bargain, until you open the crate and find that only one orange in ten is fit to eat. You didn't buy cheap oranges. You bought very expensive ones, packed with rot.

A low CPM can be that cr...