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To test the effectiveness of two Web advertising agencies you
To test the effectiveness of two Web advertising agencies, you increase your ad purchase with agency A by 50% without changing your purchase through agency B. The referrals to your Web site from agency A increased by only 34% but the referrals from agency B fell by 21%. Why might the difference-in-difference estimate of the referrals per dollar through agency A be biased?

To test the effectiveness of two Web advertising agencies you