Do Ads work?

If your ads work, if you can measure them and they return more profit than they cost, why not keep buying them until they stop working?

And if they don't work, why are you running them?

The time-tested response is that you're not sure, that ads are risky, that you can't tell. And for some sorts of products and some sorts of ads, you'll get no argument from me.

- Seth Godin, 'Do Ads work?'