What should I measure when tracking advertising spend?

One of the most frustrating parts of advertising is that we don't always know if an advertisement actually influenced a buying decision. We thought that being able to track clicks in online ads would change this - but we were wrong.

According to Google, "any display ad seen by a consumer can have an impact on her buying behavior, even when she doesn't click." 

Advertisements often have a different impact on your customers depending on when they interact with it. 

"At the beginning of the purchase path, advertising helps customers gain awareness of your product or service. In the middle, it creates desire and boosts interest. And at the end, it helps to seal the deal."