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If that's the case then you can say the commercial is ineffective, because you can say that the commercial didn't raise sales. You can never conclusively say that the commercial raised sales when they're up or lowered sales when they're down, but the commercial surely didn't raise sales when they're down.

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For those who are downvoting me, care to explain why I deserve it? Where am I wrong?

If sales would have been -14% without the commercial but are only -7% with the commercial, then the commercial did raise sales. You measure vs. the counterfactual baseline, the one where the rest of the world is held constant but you don't take the action that you're measuring.

You seem to have missed the point of the question "After all, how do you know that sales wouldn't be down 14% without the ads?".

The commercial is not the only thing affecting sales. If there is some other factor driving sales down strongly the sales may still fall overall even if the commercial is successful (i.e. attracted a large number of sales).

I see. My point was that good advertising should increase sales not slow the decrease of sales. But I guess when you're running a business that's tanking the OP would be correct in calling the ad effective.

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