The average CTR across ads on the web is 0.06% (see the Smart Insights article link for more detail), so a basic performance benchmark to measure your ads against would be about 6 clicks (leads) per 10,000 impressions. https://www.smartinsights.com/internet-advertising/internet-advertising-analytics/display-advertising-clickthrough-rates/

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We typically use customer surveys to determine what percentage of our customers shop in our stores as a result of various advertising methods and other reasons. Based on these surveys, we estimate the return on each method of advertising (including outdoor advertising like billboards) based on the average profit per customer and the percentage of customers reporting that their visit resulted from a particular advertising method.

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