How effective is online advertising really? Mere clickthrough statistics clearly won't answer that question.

I'm often puzzled at top search ads promoting the exact same brand name I searched for. Apple's App Store does this too. Apple's motivation to display the ads is obvious, but it's hard to see what the advertiser gets out of it.

Then again, I do believe advertising works in general, even if I don't like it in most cases. Online advertising should also work in the same way street advertising works. Creating simple awareness, association, or reminding people that a brand exists, seems an important (but hard to measure) role of ads anywhere.