[pedrosorio]

If you chose "not to show ads" to the control group, all the slots of the control group where you would have shown ads will have ads you don't control, including your competitors' ads.

If you show a neutral ad, you know exactly how much seeing your brand in ad drives revenue compared to seeing an irrelevant ad.

[m3047]

Hey Jefftk, this is a human dimension I wasn't acknowledging elsewhere. Theory is one thing and money is another. ("whiskey is for drinking, and water is for fighting over" or something like that -- Samuel Clemens)

So in the real world I bet this is what you're talking about: "but there are clickmonkeys out there" or words to that effect.

[edit:] But to be honest, I'm not sure it directly answers the question about entanglements between ad and no ad.